You have ticked the checkbox in Plex “Manually specify public port”.
This is for the case when you create the port forwarding manually in the configuration of your router.
UPnP on the other hand is picking a random port number automatically every time. Therefore you don’t tick the checkbox, if you want to continue using UPnP.
However, most routers nowadays are disabling port forwardings which were created by UPnP after a certain amount of time.
It is therefore recommended to create a port forwarding by hand, tick the checkbox and enter the public port number behind it.
It is recommended to NOT use the Plex default port number of 32400 for the public (WAN) side. Instead, use a random number between 20000 and 50000. But keep the internal (LAN) port number always at 32400. Only forward TCP packets.
