Match the symptom, then work through the safe checks.
1. What best matches what you observe?
2. Complete only the checks that are safe for you.
Show stop conditions
- The next step would require disconnecting, cleaning, cutting, bending, or inspecting the fiber end
- The cable is crushed, cut, sharply kinked, or the wall/ONT optical connector is damaged
- The provider needs an optical-power reading or outside-line test
Likely causes
- The provider optical signal is absent or below the ONT's usable range
- The fiber cable is sharply bent, crushed, disconnected, or damaged somewhere in the path
- A connector, splitter, outside drop, OLT port, or provider-side fiber path has a fault
- The ONT is not ranging or authenticating after an optical-link interruption
Quick checks, in order
Do not look into, unplug, or clean the fiber connector
Optical light can be invisible and fiber connectors are easy to contaminate or damage. Keep the connector seated unless your provider explicitly instructs otherwise.
Inspect only the visible fiber route
Look for a crushed section, sharp bend, loose wall box, or obvious physical damage without pulling on the cable or opening provider equipment.
Restart the ONT once only if your provider permits it
Power-cycle the ONT normally and allow several minutes for PON ranging and authentication. Do not factory-reset it.
Contact the fiber provider if LOS stays red
The provider can measure optical power and test the OLT, splitter, drop fiber, and ONT. Persistent LOS is not normally fixed by changing Wi-Fi settings.
What do the optical indicators show?
This strongly points to an optical-link problem. Nokia documents PON-off with fiber not connected or link failure (LOS/LOF).
On supported TP-Link GPON terminals, flashing red LOS means no optical signal is received or the received signal is too weak.
The optical link may be established; move the diagnosis to router WAN, account/session, DNS, or provider routing rather than the fiber light.
Stop and get qualified help when
- The next step would require disconnecting, cleaning, cutting, bending, or inspecting the fiber end
- The cable is crushed, cut, sharply kinked, or the wall/ONT optical connector is damaged
- The provider needs an optical-power reading or outside-line test
Official support and model manuals
Use the full model number from the rating label. The manufacturer manual is the deciding reference when codes differ by region or product family.
Frequently asked questions
What does LOS mean on a fiber modem or ONT?
LOS commonly means Loss Of Signal. On supported ONTs, a red LOS or alarm state indicates that the expected optical signal is missing or outside the usable range.
Can restarting the router fix a red LOS light?
A restart can recover a temporary ONT state, but it cannot repair a broken, disconnected, contaminated, or weak optical path. If LOS remains red after one permitted restart, contact the provider.
Should I unplug and clean the fiber connector?
Not as a normal user check. Fiber connectors are contamination-sensitive and the optical signal can be invisible. Leave the connector seated and let the provider test or clean the optical path.