Home Internet - Port Forwarding not working

I was able to get port forwarding to work. In my setup I use the T-Mobile Gateway → USG-3P → Home Network. In this case I have a router behind another router. In the home network I have a Plex Media server. Here's the steps I used to get the Plex Media server to talk to devices outside (Internet) the home network.

1. Change the IP address of the T-Mobile gateway from 192.168.1.1 to 192.168.1.2
2. I left the USG-3P address range at 192.168.1.x
3. Enabled port forwarding on the USG-3P so that 32400 would reach my Plex server. Essentially nothing changed on the USG-3P, this was what I had when it was connected to a Comcast modem.
4. On the T-Mobile Gateway I enabled the DMZ and put the USG-3P into it.
5. I disabled all Wifi capability of the T-Mobile Gateway, and only used a single LAN connection directly into the USG-3P.
6. I disabled ALL firewall settings on the T-Mobile Gateway. I figured this is okay because the only outlet is directly into my USG-3P router that has all it's firewall enabled. Again, all Wifi is disabled on the T-Mobile Gateway and the only outlet is a LAN port directly into the USG-3P which firewalls my home network.
7. I set the virtual server routing on the T-Mobile Gateway for port 32400 to point to the USG-3P.

After that I was able to successfully get devices on the external Internet to talk to my Plex server. Logically it seemed that I would only either need to enable DMZ or Virtual Server, and it seemed like I shouldn't have to disable all the T-Mobile Firewalls, but it wouldn't work unless I did all the above steps. Again, I would not recommend this unless you have a dual router situation, you disable Wifi on the T-Mobile router, and you place your entire home network behind a second router that has the proper Firewall protections.