I use T-Mobile 5G Gateway to connect to internet. I’m not clear if AX1800 Mesh WiFi 6 System will wo

You could connect another router to the tmobile gateway. 

The gateways tend to have issues with being put in passthrough/modem only mode though. 

If you connected another router to it, you'd either want the 2nd router in access point mode or putting the 2nd routers IP address into the tmobile's dmz.