Netflix no longer streaming

So, the logic behind connecting the clients with the delivery issue direct to the gateway is very simple. IF the delivery is via IPV6, which it is quite possible that is the case then you would have to make sure that traffic is forwarded across YOUR router or mesh gateway interface via IPv6 or it simply will not pass. If you connect the TV or Firestick or whatever it is to either an interface on the network direct from the T-Mobile gateway you remove the additional variable which is your router. When you use the additional router solution you establish a double NAT scenario and may not be routing IPv6. 

If you have left the wireless on the T-Mobile gateway enabled join the TV or Firestick etc… to the wireless network established by the cellular gateway. It does handle IPv4 and IPv6 traffic. I can clearly see this with my Nokia gateway and the statistics reflect multicast traffic TX/RX. I run clients dual stack with IPv4 and IPv6 with no issue. If you want to know your external IPv4/IPv6 address go to whatismyipaddress.com and check it out. If you have a client on your local network and want to know if it can communicate out across your router issue an IPv6 ping to the external IPv6 address from the client or issue an IPv6 ping to one of the major DNS hosts on the internet. If your client cannot communicate out via IPv6 from your local router network do the same test via connected to the network established via the T-Mobile gateway. 

You have nothing to lose by testing the TV or whatever client on the T-Mobile gateway network. It might just work and reveal there is a communication problem across your router.