disable UPNP.
stick to one port instead of swapping around.
restart plex after you pick a port and have it setup properly.
check your port on canyouseeme.org for success.
When I captured network traffic, it seemed the three seconds was the amount of time it allowed to see packets from the Plex servers to verify that inbound traffic was being allowed. No packets in 3 seconds, then it would report not accessible. The 3 seconds of “connect” causes confusion. Similarly, sometimes remote access is actually working though what I’m seeing is no traffic is coming from these hosted Plex validation servers.
What happens when you go to canyouseeme.org and put in the external/internet facing port that you have configured? (Note - this external port must be the same in both the router’s port forwarding rule and in the Plex manually specified public port.)
I am having the exact same issues. CONNECT for only a few seconds. Got much worse after latest update yesterday. Plex user from the very beginning. Nothing changed on my end. IT guy too.