how to connect orbi mesh router to T-mobile gateway

The Orbi startup guide recommends disabling the WI-FI on your modem/gateway prior to connection of the Orbi router to the modem/gateway. They state to use the Ethernet cable to connect the Orbi to the modem/gateway using the yellow internet port on the back of the rbr50. Pretty straightforward process. The Orbi router can be managed via the mobile application OR with a browser. Just for convenience sake I would suggest connecting your laptop or client with an Ethernet cable to one of the three Ethernet ports on the back of the Orbi. With the browser you enter the orbilogin.com address in the URL window at the top of the browser. IF you have already done the initial setup of the Orbi system with another modem and you have not defaulted it to factory defaults you should be able to use your existing administrative login information. When the home page renders then you would select "Internet" to goto the setup options for the internet connection. You need to confirm the setup of the IP address is by DHCP not static as if it was set to static for a prior solution that will now work with the T-Mobile gateway. You can also set the DNS servers OR allow it to obtain the DNS from the T-Mobile gateway and it should be provided 192.168.12.1 for DNS resolution. For the default MAC address I would suggest to just use the default for the Orbi rbr50. Once you have these basic parameters you can then "Apply" them and the basic requirements are met for the connection to the modem/gateway. 

ALL the above is in the Orbi Installation Guide which is readily available on the internet so the step by step recommendations are all easy to come by. The key with the T-Mobile gateway you have for turning the wireless off is a little more to consider. This will depend in part on the gateway model you have and the process required to manage the gateway. If you have the Arcadyan or Sagemcon gateway you will need to have the T-Mobile home internet mobile application installed on your phone. If you find there is no option to turn the wireless off then it is time to visit the Nater Tater YouTube video collection, find the video he has done on the respective T-Mobile gateway for the backdoor method to turn the wireless radios off on the gateway. With either one it is not super difficult but does require a little focus. It helps to have some technical expertise but with patience and just a bit of effort it can be done. There are others on the community that have also done this and I am sure you can get more answers and help along the way. 

I have seen some users reporting that throughput on the Ethernet port of the T-Mobile gateway can produce very poor throughput result. Since it appears that some of the gateways have Ethernet modules that do not perform very well I suggest connecting a laptop or client with an Ethernet cable at the very beginning, before connecting the Orbi router, to confirm the operation of the Ethernet ports on the gateway. The T-Mobile gateways commonly have a couple of Ethernet ports on the back. You can run a speed test with Speedtest.net or say speed.cloudflare.com or any other internet speed test you may prefer and confirm a baseline of what to expect. The Ethernet ports are supposed to be gigabit capable but the determining factor for throughput is going to be the cellular frequency (signal) between the T-Mobile gateway and the cell tower. If the speed test result is good say triple digit that is a good start. If the speed test only reports single or double digits well confirm the T-Mobile gateway is in the optimal location for its communication to the cellular signals.  After you run a speed test with the Ethernet wired connection do the same with the same client on a wireless connection vs using the wired connection. If your wireless connection provides a better response for speed than the Ethernet wired connection that is off to a bad start. That would suggest to me the Ethernet module might not be functioning very well and if that is the case all traffic from the Orbi router would be subject to the poor performance of the Ethernet connection due to the Ethernet connection on the T-Mobile gateway. If that does happen contact T-Mobile support and report the behavior and see if you can get a properly functioning gateway. If the speed testing from say a couple of wired clients is consistent over the Ethernet port and a wire then I would say the common denominator is the T-Mobile gateway Ethernet port. Just a heads up that this might be an issue to be aware of and test before spending a bunch of time with the router to gateway connection with the Ethernet cable.