The remote access going green initially is a red herring. It will always go green after you make a change, while Plex is actually checking the access. Once the check is complete it correctly changes to red. It is not actually briefly working and then stopping.
Please read through Remote Access and Troubleshooting Remote Access support pages for how to get your remote access setup.
If you are still having trouble then step through this guide: The Basics of Remote Access Troubleshooting
If you have been unable to get remote access working after you have completed all of the above (please be thorough and don’t skip any steps, or think you’ve already done all of this so you don’t need to bother), come back and we’ll get some details from you and proceed from there.
