Trying to get our new Gigabit service working. Ran an ethernet cable from ONT to modem - checked cable with cable tester - Ethernet does not show as active - no lights on the connector. Internet is still functioning through coax at 100Mbps
What model is your router (you called it modem presumably.)?
You need to disable WAN Coax on your router (Assuming G1100 or older) via My Networks -> Network Connections -> Broadband Connection (Ethernet/Coax) -> Settings -> Enable/Disable Coax Link: Disable. Then the WAN Ethernet will be connected? I have not experimented with my router, so hopefully this works.