To bring this thread to a conclusion, there are a few things that can cause the ARC app to lose visibility to your server even when port forwarding is correctly configured:
If you’ve migrated your Roon Server between machines, then ARC will still look for the server on the first machine. You’ll need to disconnect from the server and resync or redownload the app.
If you’ve restored from a Backup, then ARC will treat the newly restored database as a completely new install. You’ll also need to reinstall or disconnect/resync
Lastly, if ARC hasn’t been able to connect with the server in several weeks (offline mode or no network pathway), the session will become stale. You’ll need to reconnect ARC on your home network again to resync, but you likely won’t need to reinstall.
The team has work in the pipeline to make these transitions less disruptive and offer a cleaner user experience for connectivity in ARC overall.