As a follow-up, I was able to reclaim my server by following these steps:
- Download a Firefox docker via UNRAiD/Apps console, put the docker on the same network as my Plex instance
- Navigated to http://my.ip.add.ress:32400/web via the browser
- Accessed server settings, authenticated and was able to reclaim the server.
I hope this helps some of you experiencing the same issues.