If it’s „working“ for a few seconds, consider it’s not working at all. That’s probably just Plex assuming it’s green while it’s testing the remote connection.
Did you already have a look at the related troubleshooting support article?
https://support.plex.tv/articles/200931138-troubleshooting-remote-access/
There’s also a guide with some more details on how to troubleshoot remote access issues.
