Don’t. 3rd-party Uninstallers and “Registry cleaners” make things worse.
The “thorough” uninstallation procedure is here:
https://support.plex.tv/articles/201941078-uninstall-plex-media-server/
Though your issues might have more to do with a wonky internet connection or a slow DNS server: Cannot find Server - #2 by OttoKerner
