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.
You were correct, thank you so much. The apps were in the default network docker0. I looked into it, and the easiest option for my situation was simply to create another bridge network and move my apps into that. Now readarr can find Host = Calibre.
Yes, I’m using hub.docker.com and portainer. The other 7 or so containers are in bridge mode on the network I set up. Plex, per the linuxserver.io docs, needs to be under network mode host.
I have set port forwarding on my router to allow external access to port 32400.