I've said before that I've spent almost 50 years in some trade or another where troubleshooting and repair were/are a big part of what I did/do. Everyone in my place has made things work and had no idea what really fixed the problem. Never trust any service person that says otherwise.
So. Yesterday my router stopped seeing the internet. Reset both the router and the cable modem. Got with COX this morning and they said they could "see" my modem and everything looked fine. I ran the router troubleshooting app and it said everything was fine with the router. I checked the connections and tested the cables connecting everything. Nothing worked.
So I decided to redo everything. The modem used to be right by the coaxial wall jack, then a 20 ft Cat 6 cable to the router. I changed to a 20 ft coax cable to the modem which is now right next to the router. Everything works now but I have no idea what fixed it.