Recently after installing a new router, I ran into an issue where my Plex server was not accessible from the internet.
The way Plex is setup to work seems to be like:
Internet --> External-IP:random-port --> Internal-IP-of-Plex-Server:32400
Originally, I had forgotten to enable port-forwarding on my router for port 32400. The solution seemed simple, enable that port (32400) to my Plex Server’s internal IP and while at it, also enable that random port (which didn’t seem so random at the time). That was a problem.
Enabling the both port causes a problem (not really sure why). So I looked at the option on Plex to set the external port to 32400 with no luck. The solution was the simplest one; only enable port forwarding for port 32400 at the internal network level. Meaning, just turn on port forwarding for port 32400 to the Internal-IP-of-Plex-Server. Leave the rest alone. Do not set an external port and do not enable port forwarding for the random port (even if it does not seem random).