Are you able to see & read/view the media files at all? And
x-www-browser is just a link to a regular web browser (firefox here), but apparently that needs Avahi to be working. And the help webpage below says the link may really be
http://localhost:32400/manage/index.html It should work from any connected computer too (replacing localhost with the computer's ip). But running your web browser with sudo shouldn't affect the results.
After browsing https://forums.plex.tv/index.php/topic/26727-how-to-plex-media-server-on-ubuntu/ it appears that Plex runs as it's own user, so maybe the plex user doesn't have read permission on the media files...
If you want to quickly and temporarily add a second mount point (to test if plex will see the media files mounted somewhere else) you can bind mount it to another directory with
mount --bind olddir newdir (kind of like "linking" the two directories)
Then to change the mount point permanently just edit it's line in
/etc/fstab changing the old /media/radibg2/Radi/ into /media/Radi (FYI
lsblk will tell you what partition is currently mounted where).