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.

 

 

Something to ask that ISP about. 

 

Something to keep in mind as well:

Your ISP Modem already has a built in router and wifi. This would be a double NAT (two router) condition which isn't recommended. https://kb.netgear.com/30186/What-is-Double-NAT
https://kb.netgear.com/30187/How-to-fix-issues-with-Double-NAT
Couple of options,
1. Configure the modem for transparent bridge or modem only mode. Then use the Orbi router in router mode. You'll need to contact the ISP for help and information in regards to the modem being bridged correctly.
2. If you can't bridge the modem, disable ALL wifi radios on the modem, configure the modems DMZ/ExposedHost or IP Pass-Through for the IP address the Orbi router gets from the modem. Then you can use the Orbi router in Router mode.
3. Or disable all wifi radios on the modem and connect the Orbi router to the modem, configure AP mode on the Orbi router. https://kb.netgear.com/31218/How-do-I-configure-my-Orbi-router-to-act-as-an-access-point and https://www.youtube.com/watch?v=H7LOcJ8GdDo&app=desktop
Try option #2 first...
https://kb.netgear.com/000061277/Which-features-are-disabled-on-my-Orbi-router-when-it-is-set-to-AP-Mode