I have a similar issue on a Mac Mini 2010 server. In my situation, the fan runs high and the ethernet port doesn’t seem to work. For some reason, I can’t run any diagnostics on it. Most likely it is a motherboard issue.
I had discovered it was my router that was causing an issue. I have a Synology router and it has a setting called Threat Prevention. It was TP that was blocking any traffic through it including traffic to/from the Ring Doorbell to my phone. Once I excluded the Ring Doorbell, the problem was fixed.