I just installed Plex server to try this. Intentionally disabled UPNP NAT-PMP because I don’t like it, and forwarded port 32400 through from WAN to LAN IP.
Didn’t work. Tried a manual port of 32401 didn’t work. Lowered manual public port to 444 as a test and it did work.
Then I came back to the screen again about 30mins later and it had stopped working again.
Playing a hunch, I tried a different laptop and it said that Plex was fully accessible outside of the network - green light.
I think Plex server is a bit crap at coping with port changes in general - that it takes a few attempts to connect on a new port before it will work, and that the web UI has browser session dependencies that screw with you when you’re fault finding.