Port forwarding is just doing manually what UPnP tries to do automatically. If you go to canyouseeme.org and enter the port that it says is being used next to your public IP in the remote access settings does it show as being accessible?
Can you access it remotely from a browser? turn off wifi on your phone so just on cellular and go to https://app.plex.tv in browser to check
Here is some basic info on how to port forward in a router but I don’t know what router you have so things may look different. [How-To] Port Forward Asus Router
