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If your internet is still down after a fiber installation, first confirm that the service is activated, then check the optical network terminal (ONT), the assigned Ethernet port and cable, the router’s WAN port and settings, and finally whether the fault is limited to Wi‑Fi. This sequence isolates an ISP provisioning problem, a fiber/ONT fault, a router connection problem, or a single-device issue without changing settings that belong to another provider.
What should I check first when fiber internet is not working?
Start with the installation or activation message from your internet service provider (ISP). Verify that the activation date and time have passed and that the provider has told you which ONT LAN port to use. Then work outward from the fiber equipment: power and optical lights, Ethernet cable, router WAN port, router configuration, and a wired test device.
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Do not assume that every fiber installation uses the same port, light meanings or WAN settings. The correct port and configuration depend on the ISP, network and equipment supplied at your address. For example, one provider may specify ONT LAN1 and DHCP, while another may require PPPoE or a VLAN. Use your provider’s instructions rather than copying settings from a different service.
1. How do I confirm that the fiber service is activated?
A newly installed fiber line can be physically complete while the broadband account is still waiting for activation or provisioning. Check the provider’s email, text message, customer portal or appointment paperwork for an explicit activation confirmation.
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- Find the stated activation date and time. If it has not arrived, wait for that point before changing router settings.
- Look for a message saying that your broadband service is live, registered or ready to connect.
- Check that the account, service address and installation reference match your order.
- Read the connection instructions for the exact ONT and router supplied by your ISP.
- If activation was promised but is not shown, contact the ISP and ask it to verify provisioning before replacing equipment.
Enable Networks’ activation guide specifically instructs customers to connect after activation is confirmed and to use the ONT LAN port assigned by the provider. The assigned port is not safely assumed to be identical on every installation (Enable Networks’ activation guide).
An activation problem cannot be fixed by rebooting a router repeatedly. Ask support whether the ONT is registered to your account and whether the service has been enabled on the correct port.
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2. What do the ONT power, PON and LOS lights mean?
The ONT is the fiber network termination equipment inside your premises. It receives the optical signal from the fiber network and normally hands the connection to your router through Ethernet. Check its power and indicator lights before investigating Wi‑Fi.
- Confirm that the ONT power adapter is connected to the ONT and a working electrical outlet.
- Check whether the power light is on. If it is dark, test the outlet with another appliance and verify that the adapter is firmly seated.
- Read the labels on your particular unit. Common labels include PON or Optical, LOS (loss of service), LAN and Power.
- Compare each light with the ONT manual or your ISP’s support page. Meanings vary by model and network.
On Openreach equipment, a solid green PON light with the LOS light off indicates that the ONT is working properly. Openreach says that a PON light that is off or flashing, or an LOS light that is red, can indicate a network issue and directs customers to their communications provider. That interpretation applies to the cited Openreach equipment, not automatically to every ONT (Openreach’s fibre termination-point checks).
| Observation | What it can indicate | What to do |
|---|---|---|
| Power off | No electrical power to the ONT, a loose adapter, or failed equipment | Check the outlet and connections; contact the ISP if power will not return |
| PON/Optical normal for the model | The optical link may be established | Continue to the Ethernet and router checks |
| PON off or flashing where the guide says it should be steady | Possible registration or fiber-network problem | Record the state and ask the ISP to check the line and ONT registration |
| LOS red, or an equivalent fault indication | Possible loss of optical signal | Do not attempt fiber repair; contact the ISP |
| LAN light off with a connected router | No Ethernet link, wrong port, bad cable or powered-off router | Check the assigned ONT port, cable and router WAN port |
Do not unplug or replace the optical connector, open the ONT or buy a self-configured PON device as a routine remedy. Optical equipment can be registered to the provider’s network, and a replacement may not authenticate.
3. Is the Ethernet cable connected to the correct ONT and router ports?
The fiber terminates at the ONT; the router normally connects to the ONT with an Ethernet cable. A cable in the wrong ONT socket can leave the optical lights normal while the router receives no service.
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- Locate the ONT LAN ports and identify the port named in your ISP’s installation instructions.
- Push one end of the Ethernet cable fully into that designated ONT port.
- Connect the other end to the router’s WAN, Internet or sometimes blue-labeled port—not one of the numbered LAN ports used for local devices.
- Check for a link light on the ONT LAN port and the router WAN/Internet indicator, if your equipment provides them.
- Reseat both ends. If the cable is missing, visibly damaged or fails to create a link, substitute a known-good Ethernet cable of suitable length.
Plusnet’s Openreach example uses ONT PORT1/LAN1 to the router’s WAN port, but that is a provider-specific setup. Follow your own ISP’s port assignment rather than treating LAN1 or PORT1 as universal (Plusnet’s full-fibre connection guide). Enable Networks likewise emphasizes using the provider-designated ONT LAN port (Enable Networks’ activation guide).
A replacement patch cable can help only when the original cable is missing, damaged or fails a known-good substitution. It cannot restore a missing optical signal, activate an account or correct an ISP-side configuration.
4. How can I tell whether the problem is Wi‑Fi or the whole fiber service?
Compare a wired device with a Wi‑Fi device. A working wired connection means the fiber service and router’s internet path are probably available, while a Wi‑Fi-only failure points to wireless settings, coverage or the device itself.
- Use a laptop or desktop with an Ethernet socket, or a suitable Ethernet adapter.
- Connect it to one of the router’s numbered LAN ports, not the router’s WAN port.
- Turn off Wi‑Fi on that device temporarily so it cannot silently switch back to wireless.
- Open several ordinary websites or use the connectivity check recommended by your ISP.
- Compare the result with a phone or other device connected over Wi‑Fi.
| Wired result | Wi‑Fi result | Likely area |
|---|---|---|
| Works | Fails on one device | That device’s Wi‑Fi, DNS, VPN, proxy or network profile |
| Works | Fails on all wireless devices | Router wireless settings, radio state, coverage or a router fault |
| Fails | Fails | Activation, ONT, Ethernet path, router WAN configuration or ISP service |
| Intermittent | Intermittent | Could involve a loose link, provisioning issue, router fault or wider network problem; record times and light states |
ASH NET’s troubleshooting checklist uses the same wired-versus-Wi‑Fi distinction: wired working while Wi‑Fi fails is a different fault path from both types of connection failing (ASH NET’s no-internet checklist).
If only one device is affected, forget and rejoin the wireless network, check that airplane mode is off, and temporarily disable a VPN or proxy. Avoid changing the router’s WAN settings when another wired device already has internet access.
5. Which router WAN settings should I use after fiber installation?
Use only the WAN protocol, VLAN ID, username and password supplied by your ISP. A normal optical light does not prove that the router is provisioned correctly.
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Common possibilities include:
- DHCP or Dynamic IP: the router requests an address automatically.
- PPPoE: the router requires an ISP-provided username and password.
- VLAN tagging: the ISP requires a specific VLAN ID in addition to the WAN protocol.
- Provider-supplied router: the ISP may have preconfigured or remotely provisioned the equipment.
- Open the router’s administration page using the address and credentials in its manual or ISP instructions.
- Open the Internet, WAN or Network settings page.
- Choose the protocol specified by the ISP. Do not select PPPoE, DHCP or a VLAN value based on a generic online guide.
- Enter credentials exactly as supplied, observing capitalization and punctuation.
- Save the configuration and allow the router to reconnect. If the ISP requires a restart after provisioning, follow its stated sequence.
ASH NET documents DHCP with VLAN 10 for its own service, while Tenda notes that ONT registration and WAN configuration depend on the provider and device. Those examples demonstrate why one provider’s settings must not be copied to another (ASH NET’s checklist; Tenda’s internet troubleshooting guidance).
If you use a third-party router, ask the ISP first whether it is compatible and whether it must support VLAN tagging, PPPoE credentials, a particular MAC address or ONT registration. Do not factory-reset a provider-supplied router unless support tells you to; a reset can erase provisioning details.
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A controlled restart can clear a temporary link or DHCP state, but it will not repair a cut fiber, an unactivated service or an unknown VLAN requirement. Restart only after checking activation, lights and cabling.
- Save any work and shut down or disconnect devices that are using the connection.
- Turn off the router. Leave the ONT powered unless your ISP’s instructions specifically say to restart it.
- Wait briefly, then power the router back on.
- Wait for its WAN/Internet light and allow several minutes for it to request service.
- Reconnect one wired device and test before reconnecting every device.
If the ISP instructs you to power-cycle both units, follow its exact order and waiting times. Repeated rapid power cycles make it harder to tell whether the ONT is completing registration.
7. When should I contact the ISP instead of continuing to troubleshoot?
Contact the provider when the ONT shows an abnormal optical state, activation is not confirmed, the assigned port and cable path are correct but there is still no service, or you do not know the required WAN settings. These conditions require account, line or network checks that cannot be completed from a home router.
Escalate immediately if:
- The ONT’s PON/Optical light is off or flashing contrary to its guide.
- The LOS light is red or another optical fault indicator is present.
- The ONT has power but no LAN link appears with a known-good cable and the correct port.
- The activation date has passed but the ISP cannot show the service as live.
- Both wired and Wi‑Fi devices fail after the basic connection checks.
- The router requires credentials or VLAN information that the ISP has not supplied.
- The service worked and then stopped, especially if the ONT lights changed at the same time.
Openreach advises customers to contact their communications provider when its specified PON/LOS pattern indicates a possible network issue (Openreach’s support guidance). Plusnet and other providers similarly direct customers through provider-specific full-fiber checks rather than a universal configuration.
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What information should I give support?
Providing observations lets the ISP distinguish provisioning, optical, Ethernet and Wi‑Fi faults without repeating every step.
- Service address and account or order reference.
- Installation and promised activation date.
- ONT make and model, if shown on its label.
- Exact light states and whether they are solid, flashing, off, green, amber or red.
- The ONT port used and the router port used.
- Whether a known-good Ethernet cable was tried.
- Whether a wired device fails, whether Wi‑Fi fails, or whether only one device is affected.
- Router model and the WAN protocol or VLAN setting currently selected.
- When the problem began and whether anything changed immediately beforehand.
Do not send passwords or expose the full router administration credentials in a public support forum. Give account details through the ISP’s authenticated support channel.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why can the ONT look normal while there is still no internet?
Optical synchronization and internet access are separate stages. The ONT may have a healthy fiber signal while the service is not activated, the Ethernet cable is in an unassigned port, the router is connected to its LAN socket instead of WAN, or the WAN protocol and VLAN do not match the ISP’s requirements.
That is why the troubleshooting order matters: first establish that the account is live, then verify optical status, then verify the physical Ethernet link, then verify router configuration, and finally isolate wireless or device problems. A normal PON light is useful evidence, not a guarantee that every service layer is configured.
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No. Do not treat an ONT replacement, a self-purchased PON device, fiber connector work or a factory reset as routine troubleshooting. The ONT may need to be registered by the ISP, and optical work requires the provider or its appointed technician.
You can safely check power, read the indicators, reseat the Ethernet cable and follow the ISP’s documented router instructions. Stop and escalate if a check involves opening equipment, handling the optical connector or changing a registration setting.
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Fiber installation troubleshooting decision path
Use this short decision path after recording the light states:
- Is activation confirmed? If no, ask the ISP to confirm provisioning.
- Is ONT power present and its optical status normal for that model? If no, report the exact lights to the ISP.
- Is the Ethernet cable in the provider-assigned ONT port and the router WAN port? If no, correct it and retest.
- Does a wired device reach the internet? If yes, troubleshoot Wi‑Fi or the affected device rather than the fiber line.
- Does the router use the ISP-required WAN protocol and VLAN? If unknown, obtain the settings from the ISP.
- Do wired and wireless devices still fail? Give support the activation status, light states, ports, cable result and test results so it can investigate the service.
This approach avoids two common mistakes: changing advanced router settings when the account is not activated, and blaming Wi‑Fi when the ONT has lost its optical signal.
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How long should I wait after a fiber installation before troubleshooting?
Use the activation date and time in the ISP’s confirmation rather than an assumed waiting period. If that time has passed and the provider cannot show the service as active, ask support to check provisioning.
Can I use my old cable modem with a new fiber connection?
A cable modem is designed for a different access network and normally cannot terminate a fiber ONT connection. The router must connect by Ethernet to the ONT, and its compatibility and WAN requirements should be confirmed with the fiber ISP.
Why does my phone say Wi‑Fi connected but have no internet?
Wi‑Fi connection only proves that the phone reached the router’s wireless network; it does not prove that the router reached the ISP. Test a wired device and check the router’s WAN status to separate local wireless access from an internet-service fault.
What does a red LOS light mean on a fiber box?
On equipment where LOS means loss of service, a red light can indicate that the ONT is not receiving the expected optical signal. Light meanings vary by model, so record the state and contact the ISP rather than disconnecting or repairing the fiber.
Should I buy a new router if fiber internet is not working?
Not before the ISP confirms that the router is compatible and identifies the required WAN protocol, VLAN and credentials. A new router will not fix an unactivated account, an optical fault or an Ethernet cable plugged into the wrong ONT port.
