I have Plex on a Netgear ReadyNAS Server. I had to completely reset my router last night.
I have not been able to get my Media Server back.
At first I was getting the Not Authorized error. I went into the Preferences.xml file and deleted the items that were suggested online. When I rebooted it set up a new server under a different name but with all the same files as well as the original server now says it is unavailable.
If I read your comment correctly you have 2 servers, the “New” server, and the “Old” server. If that is the case, go into the authorized devices in settings and remove the “Old” server.