OK Update.
One mistery solved. The NON-Working automatic port was indeed user error by me.
I forwarded 21034 public to 21034 intranet. That cannot work of course. I removed the manual port forwarding entirely. No manual port open for plex anymore. Now UPnP has taken over again and that works. And i have acces over internet again as well.
I will now leave it like that for a few days and report back if its all good now. So you where right my manual port forwarding did screw something up additionally and blocked UPnP.
That does not explain the disconnects after a few days that I experienced from November till January though. Maybe it was a quirk of the ISP, maybe an issue with plex servers. I don’t know. But an ISP issue affecting only a very specific port and service seems unlikely to me.
