Firewalla Purple OpenVPN issues with t-mobile 5g gateway

I currently have a T-mobile 5G gateway connected to my Firewalla purple. The Firewalla is in router mode connected to 3 eeros in bridge mode.

I'm having issues with setting up my Firewalla VPN through OpenVpn as it keeps timing out during setup. Following the instructions here I believe I'm double NATed due to my public IP and Firewalla IP being different - however I'm not sure if:

a. the double NAT is truly the issue here

b. If I am double NATed caused by the T-mobile gateway, is there a way to solve this?