In order to forward a port for Plex Media Server, you'll need three main pieces of information:
- WAN/External Port: Port 32400 (TCP) is default, but you can generally use any available port in the 20,000 to 50,000 range.
- LAN/Internal Port: This will always be 32400.
- IP Address: The local IP Address of the computer running the Plex Media Server. This is what you did above.
To me this says that the WAN and LAN ports would be the same regardles of whether you use the default 32400 or pick your own port for Plex to use. Is there a possibility that the 32400 port might also be used by some other application on the computer you're running Plex on? You could try configuring Plex to use a different port and then forward that port. There's a lot of info on the page I referenced above about port forwarding options in Plex so you may want to read all of it.
The only time it mentions having different WAN vs LAN ports is if you have multiple Plex servers that are all using 32400. Then in the router you would give each server a dedicated IP and then for that server you would maybe map 32401 on the WAN side to 32400 on the LAN side for server #1 and then on server #2 you would map WAN port 32402 to 32400 on the IP for server #2. I think this type port mapping is beyond the capability of what UPnP can do so it wauld have to be configure manually. I personally don't like UPnP so I've shut it off and I manually control any port forwarding. This way a rouge application can't open a port for remote access. I think having UPnP enabled can be a security exposure.