how to connect orbi mesh router to T-mobile gateway

I have the same router, and when I changed to T-Mobile Home Internet (also from AT&T), I just unplugged the ethernet cable from the AT&T router and plugged it into one of the ethernet jacks on the T-Mo gateway.  Then I rebooted the Orbi, and everything worked perfectly.

I had been using the Orbi in router mode, as I was using the AT&T router in bridge mode. The TMHI gateway can't be used in bridge mode, and I didn't want another layer of NAT (network address translation) to be added by the Orbi, so I set the Orbi into AP (access point) mode. Some people on the forums reported problems with keeping the Orbi in router mode.

Some months later, I finally had enough of the severely limited configuration allowed by the TMHI gateway, so I put my Orbi back into router mode, but I then enabled IPv6 passthrough on the Orbi. That can sometimes speed up uploads  (or, rather, keep the Orbi from slowing down uploads below the TMHI gateway's normal upload speeds.  I did a number of speed tests going through my Orbi and also connecting directly to the wifi on the gateway, and my speeds were virtually identical.

Adding the extra level of NAT is not a problem for me, as I am not a gamer, but if you are a gamer, then that extra level of NAT might negatively affect your gaming. There are other scenarios where the extra NAT might cause problems, so I suggest you try it both ways-AP mode and router mode-and see how they work for you.