Goodmorning from the Netherlands.
I have te following issue on all my apple IOS15, TVOS 15 and a older TVOS14 devices.
When starting plex my media servers can’t be reached. I need to start up plex in a browser goto Authorised Devices, remove the tvos or ios entry. Open plex again on that device en signin with the Link | Plex, once I done that I see my mediaserver. Once I close plex on that device I have to redo the same steps as I described. Never had this issue before ios15 came out, but now olso with an older IOS14 apple tv.
I tought having 2 mediaservers on 2 different NAS would confuse plex, so removed one mediaserver and coppied my folders to the other nas, but problem is still there.
Did someone heard of this issue before, as I can’t find any solutions on the internet 
Thanks ,
greeting Patrick