The „green remote access status switching back to red within seconds“ means nothing. That’s just Plex listing the status as „green“ while still testing the connection.
As for re-doing your port forwarding etc… have you verified your setup is visible/accessible from outside your home network (e.g. using a service like canyouseeme.org?
While doing that… does the public IP recognized by canyouseeme.org match that listed in the Plex Media Server‘s remote access settings and your routers WAN IP?
