What plex container are you using? I would recommend the linuxserver one. I believe you may be running into this:
If you want to run the container in bridge network mode (instead of the recommended host network mode) you will need to specify ports. The official documentation for ports lists 32400 as the only required port. The rest of the ports are optionally used for specific purposes listed in the documentation. If you have not already claimed your server (first time setup) you need to set PLEX_CLAIM to claim a server set up with bridge networking.
EDIT: I misread your post, you’re using network_mode: host?
Yeah I really don’t get why people try to act like Valve doesn’t provide a ton of value to its customers. So much so that their “competitors” have basically said before they aren’t even going to try to compete on features or merit, because it would cost too much/they are too far behind at this point.