When sync suddenly dies after working for months I look at the network first :) A captive portal or a parental control wall on the router can silently block the servers even though Wi Fi shows connected which and yeah try a different network such as a phone hotspot or a work network to see if it completes. If it works elsewhere the reader is fine and the home network needs an adjustment.
Switch your router to standard DNS and turn off private DNS or any VPN on the reader for a quick test. I chased this for an hour switched to a hotspot and it synced instantly went back home and still nothing until I changed DNS then it all worked. Worth a try.
On the device disable battery saver or data saver and allow the app to use background data. Keep the screen awake while you hit refresh and give it a full minute. If your router offers both bands try 2.4 instead of 5 since some readers drop the higher band through walls. A small change that often stabilizes sync.
