Change private IP network address from 192.168.12.x

Yes, all that may work but double NATing comes with its own problems. And not being able to set up reserved ip assignments for things like printers and switches is a problem. And the security issue still persists if the whole world knows that these gateway devices are limited to a single /24 subnet that can't be changed.

Very amateur and low tech. Even the most basic routers/gateway devices have this capability. 

@JanusNemo unfortunately, the gateway does not allow for IP changes. When I first got mine, I wanted to do the same. But I learned that the gateway IP is static. So, I made sure all of my devices are DHCP and let the gateway spool out IP addresses. An alternative is to buy a wifi router and point your devices to the wifi router. Then connect the gateway to the router so that the router gets its internet access through the T-Mobile gateway. You can change the wifi router IP address schema to what you like. 

I hope this helps.

Agree that the T-Mobile Gateway is by far the least configurable WiFi router I have ever worked with.  No routing, port forwarding, DHCP allocation, DMZ, etc.  It's basically a plug-n-play device for the common masses.

Even the Verizon home gateway has an expert mode where you can change these kind of settings, most disappointing.