How to use High-Speed Internet Gateway with Existing Router

I wouldn't put the 2nd router in AP mode because I wouldn't want the T-mobile gateway issuing the addresses. I want the second router to give out IP addresses. You can't control those or do anything with that "router". It's more like a switch that can issue addresses since there's almost no other options. It's barely a router. I would only want the T-M gateway to provide internet and do nothing else.