No network connection after connecting fiber optic box and router

No network connection when a fiber optic box is connected to a router is often caused by configuration mismatches, PPPoE/bridge mode issues, or hardware incompatibility.Check Physical ConnectionsEnsure the fiber optic cable is properly connected to the ONT or fiber modem, and that the Ethernet cable from the ONT goes into the router's WAN port. Verify that all connectors are clean and undamaged, as dust or bent fibers can prevent traffic even if link lights are on . If using SFP modules, confirm they are compatible with both the ONT and router .Verify Router ConfigurationBridge Mode vs Router Mode: If the ONT is in bridge mode, the router must handle PPPoE authentication or static IP assignment. Incorrect WAN settings can prevent internet access .WAN Connection Type: Set the router to the correct connection type (PPPoE, Static IP, or Automatic IP) as required by your ISP. Using the wrong type can result in “connected but no internet” status .MAC Address: Some ISPs require the router's MAC address to be registered. Clone the MAC address of a previously working device if necessary .Test with Static IPAssign a static IP to a device connected directly to the ONT. If it cannot ping external addresses, the issue may be with the ONT or ISP configuration rather than the router .Check DHCP and PPPoEIf using PPPoE, ensure the username and password provided by the ISP are correctly entered.If using DHCP, confirm the ONT is passing IP addresses correctly. A failure here can indicate a misconfigured ONT or ISP-side issue .Restart and FirmwarePower cycle both the ONT and router for at least 1–2 minutes.Ensure the router firmware is up to date, as some fiber connections require the latest firmware to handle PPPoE or VLAN tagging .Advanced TroubleshootingPing Tests: Try pinging the ONT, ISP gateway, and external IPs to isolate where traffic stops.ARP Table: Check the ARP table on your computer to see if the router or ONT is visible.OTDR or Signal Testing: For long fiber runs, test signal strength to rule out attenuation or fiber damage .SummaryThe most common causes of no network connection with a fiber optic box are incorrect WAN settings, bridge mode misconfiguration, or incompatible SFP modules. Start by verifying physical connections, then check router WAN settings, PPPoE credentials, and DHCP assignment. If the problem persists, contact your ISP to confirm the ONT configuration and ensure your router is compatible with their fiber service .