Posting this just in case anyone else might run into the same situation and save them a few days of troubleshooting. So, about a week ago I updated the firmware on my router, I do it fairly often when a new security patch comes out. About a day after doing the update I could no longer access the Pi I use for feeding, no web or ssh access. I rebooted the Pi and nothing changed, pulled the SD card and checked it on my PC and couldn't find any problems. Rebooted the router. Even tried creating a new SD card to start from scratch, nothing... Then I noticed it was still feeding just fine. So then I tried some network troubleshooting and could get no pings or trace routes from my desktop PC or my laptop to the Pi, but I could ping the Pi from the router. Eventually I remembered a similar problem a while back with my wifi thermostat, it suddenly couldn't connect and I had to turn on the guest network, connect it to that, then switch it back to connecting to my normal network and after that it worked normally. So I changed the network on the Pi to my 5GHz SSID and voila, I could access the Pi through SSH and a browser again. I just left it connected to that network and it's been fine for several days. (I recently moved the Pi closer to the router, to remove about 10 feet of coax between the Pi and the antenna, that's why I wasn't using the 5GHz network before, the signal was a bit weak at the old location.) So it was some issue with the router but I have no idea what the heck it was. There were no error messages in the logs, it just stopped passing any packets from devices on the home network to the Pi.
This might be some router related bug or something that needs patched on consumer routers. 2.4 Ghz or Ethernet is the best. 5ghz doesn't have the distance or the stability through walls etc.
The old plaster walls in my house do a job on 5GHz, but since I moved the Pi and there is one less wall between it and the router the 5GHz channel is working fine. My PC on the second floor never got full speed even with an expensive dual antenna pci wifi card. I finally found a way to get an Ethernet cable into that room to solve that problem and used a switch to get a bunch of stuff off of wifi, but used up all the ports on the switch so my feeder Pi is still on wifi. I'm guessing the next firmware update for my Asus router it will probably mention some bug fix, or it was just a coincidence that it happened a day after I updated the firmware.
Just a minor data point ... I had the same issue with a Pi 3B+. Not only would it go down after a few hours -- or days -- but it would cause other issues with the 2.4GHz band. 5GHz would purr happily along. This occurred on a system with a high-ish end Peplink load balancing router and two very good APs. Signal strength from the pi was in the range of -64-69 dBm -- reasonable but not excellent. The Pi was dragging all other stations connected to that AP into the dirt. If I rebooted the Pi all would recover -- until it happened again. Solution was to replace the Pi. I did not look at the problem further but I wonder if the wi-fi TX on the Pi was emitting wide band junk.
