Port forwardings which were established by UPnP will get deleted after some time by many router models. The solution is to create the port forwarding manually, using the configuration menu of the router. Then merely telling Plex the external port number.
Thanks Otto for the advice, I wasn’t aware that the ports can get periodically cleared.
Frustrating that the ports aren’t then reestablished again via UPnP when Plex makes another remote request.
I’ve turned UPnP off and manually forwarded the port to 32400 now so will see how it goes.
I don’t have any other ports manually forwarded. Should I leave UPnP on in addition to forwarding the Plex port manually so it doesn’t affect other Internet services?