Orbi AX6000 RBR850

If this is the device that T-Mobile delivered, then the RBR850 system should simply "plug in".

https://www.t-mobile.com/home-internet/gateway 

The T-Mobile description makes it pretty clear that their intention is for customers to add Google WiFi units to create a mesh.  Using the T-Mobile device to provide internet to a separate mesh WiFi system  has several obvious issues:

• You would want to disable the WiFi6 4x4 WiFi so that it does not interfere with your own mesh system.
• Having two routers in the home network creates potential problems for specific applications, typically
  • Some types of internet gaming
  • Allowing access to resources on your network from the internet
  • Allowing VPN access to your network from the internet
  • (internet search for Double NAT will provide more information.)
• If two routers impact actual usage, then there are a number of solutions:
  • The easiest is to place the T-Mobile device into "Bridge" or "Passthrough" mode so that it performs no router functions.
  • Another solution is to define the IP address of the Orbi router as part of the T-Mobile router's DMZ.
  • Another solution is to put the Orbi router into Access Point (AP) mode, which allows the T-Moblle router to manage the entire network.
• These issues depend on how much control the T-Mobile "app" gives customers over their device.

If T-Mobile has a customer community forum, my guess is this topic will have been discussed many times.