It seems to me that the T-Mobile gateway (I have the 5G black square box model KVD21) does not allow for any kind of dhcp modifications but most importantly does not allow for Port forwarding.
I have several ports from the outside world that I need to forward to one of my internal PCs. It does not seem like the gateway automatically forwards these ports on to my local router (using ethernet WAN to connect to my router).
To me this is a massive oversight in T-Mobile's home internet rollout. Advanced users need the flexibility to set up their home network however they need to. At the very least I would have hoped that the gateway could just forward on all the traffic to my router and at my router handle it.
If T-Mobile doesn't come out with a solution soon, then I am thinking it's a deal breaker and I have to cancel their service and go back to cable :(