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

You gave us nothing to go off.
To get a hold of support. Click on "MyNetgear" at the top.
Register an account (or login) and then the device.
From there you can start the support process. Keep in mind that its only free for the first 90 days.
There's a support site with access to manuals, firmware, install guides, etc.
Go to https://www.netgear.com/support/
Type in your model number and from there it'll take you to the above.
Its a great resource to start with.

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.