Sorry for the delay. I have traveled to where the router is.
The router saves system.log and my own log on an SD. I don't see anything strange in the logs except WiFi wan interface (wlan0 radio1) is down and shortly after the logs stop exactly on the day and time the router lost connectivity. That is, I understand that from that day and hour the router absolutely crashed. Therefore the connectivity watchdogs stopped working. It also did not allow access via Ethernet cable. It is also notable that the router crashed without error messages or tainted kernel or similar.
Then the problem is clearly that the hardware watchdog has not worked. My suspicion at this point is that there has been a failure in the router's motherboard. Maybe sometimes the external USB WiFi receiver exceeds the USB power and not even the hardware watchdog works.