For those that are interested, I was successful in making my Plex server externally available whilst on a T-Mobile Home Internet connection. This process should also work in any CGNAT situation (Double NAT).
I downloaded on my Ubuntu box hosting my Plex server the app call NGROK (ngrok.com)
I created a free account, downloaded the “app”, started a TCP tunnel to NGROK over port 32400, identified the IP my custom URL was resolving to, and put that in my Custom Server URL Access field on the Network tab. The below two links have some great instructions and also identify the limitations of the free service. I am not an authority on this service, but was able to make it work in my own situation.
I hope this helps anyone looking to overcome the CGNAT configuration.
