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?
I’m pretty sure after exploring thousands of people you’re more likely to get chlamydia than rich. I could be wrong though, maybe try it?