You used: network_mode: host which would expose those on your host docker host, not the docker container. Which is why you don’t see them listed in Portainer. Did you try the IP of the computer and port 32400 in a web browser? That should load in the web browser on the same computer.
The message you’re getting in app.plex.tv is about your server port not being open in your firewall which is another issue to tackle, but from what you say here, you did everything correctly, you just need port forward 32400 to whatever IP that Plex Server is hosted, for it to say it can connect securely.