Remote access drops after connecting for a few seconds

Plex optimistically shows remote access status as „green“ while checking the status… so you should assume it’s not working to begin with.

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: