NightHawk Mesh Router MR60 not working with T-Mobile Internet

What the brand and model# of the ISP modem? 
There a built in router at the ISP modem? if so, This would be a double NAT (two router) condition which isn't recommended. This would be a double NAT 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 NG 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 NG router gets from the modem. https://kb.netgear.com/25891/DMZ-on-NETGEAR-routers
https://kb.netgear.com/24086/How-do-I-set-up-a-default-DMZ-server-on-my-Nighthawk-router
3. Or disable all wifi radios on the modem and connect the NG router to the modem, LAN to LAN configure AP mode on the NG router.

What platform in the phone? Apple Or Android? Be sure to disable any WiFi Private IP address or Random MAC address features on these devices:

https://community.netgear.com/t5/Nighthawk-App/NETGEAR-Mobile-Applications-and-Apple-Devices-FAQ/td-p/2220015/jump-to/first-unread-message

Are you getting internet access otherwise thru the system? 

What Firmware version is currently loaded?