Thanks for your insights.

The router is set up to give the PC a static IP already. I have now gone into the Networks page in Ubuntu desktop, set the IPv4 method from Automatic DHCP to Manual, and added my static IP there.

The arrs and other docker containers all use the same bridge network (and not the default network). None except for Plex are set to use Host.

I have removed Plex again and added it once more to the docker. Unfortunately Plex logs are still showing the same issue.

It says in the post that people assume they need more coffee