I managed to show the latest versions of the plex server using the omv update tab.
following this post:
I execute the following commands:
sudo rm /var/lib/apt/lists/downloads.plex.tv_repo_deb_dists_._public_InRelease
sudo apt update
sudo apt list --upgradable
From the omv website -> update management -> check
And that's it
This works. Run those 3 commands and then you can update via the update page in OMV.