tp-link router connected to tmobile gateway

That is true you don't have to disable the SSID(s) on the TMO gateway but it would just be best practice to not have the exposure nor any possible interference due to the same frequencies or overlap. In theory if there is no traffic on the radio it should not have any contention. With the T-Mobile gateway and the second Ethernet port if you have a few clients that had issues with the double NAT you could connect a gigabit Ethernet switch and just hard wire a client or two on the front end network vs the one on the other side of the NAT where the wireless clients are controlled by the tp-link router. There is usually more than one way to skin a cat so they say.