You might want to take a closer look at this troubleshooting guide in order to pin down what’s causing your issue. Plex is not yet dealing particularly well with IPv6 environments – there’s a few clients which will be able to successfully establish a remote connection to an IPv6 PMS but IIRC that’s only the iOS/tvOS clients (with some mixed results using Plex Web).
If you’re only getting a brief green status from the remote access status indicator, that doesn’t mean there’s been a successful connection. When your PMS attempts to confirm it is accessible, it’ll display a green status while that check is in progress.
