ethernet port

The ethernet ports on my Xfinity modem do not work. Yes, I've been down the usual rabbit holes - rebooting everything, swapping out hardware, with both the AI, live CS agents, and even a home repair actual person. Here's my situation:

I have my old provider still working (just in case ...). I have a somewhat complex in-house network, which includes ethernet connectivity as well as a couple of wireless setups in distant locations. All this is connected into a single Linksys router which connects via a short ethernet cable to the modem. Using the old Metronet modem (and Metronet service) everything works fine. When I switch the cable over to the Xfinity modem, the enternet is not reached. The port on the modem blinks green, while at the other end the port is solid green with orange indication of traffic. 

I have to make this work or I will have to discontinue the Xfinity service - it has not "provided" service since installation on December 27, 2025.