When can I JUST use TM internet modem as ONLY a modem, in BRIDGE mode, with NO NAT, NO firewall, and frankly NO Wifi.

To the average TMHI customer, double NAT is not going to be required. Most users will put the trashcan up, take down their old ISP provided combo modem / router device and run the same way they did with their previous ISP.  Hell, most home internet users don't even have the motivation to change the Wi-Fi password provided by the ISP installer when their home internet was originally setup. They just write down the 20 character complex password and stick it to the refrigerator with a magnet. When visitors need internet access, they just give them the password. 

As for me, I am an IT professional, and I do work from a home office, but I have no need for port forwarding. However, I do want to manage my DHCP / DNS, and have an isolated guest network. To do so, I have do use double NAT. I have no problem with that. The performance of my TMHI is still better than what i was getting form Spectrum, for 20% less cost. Couple that with the fact that I am unlikely to see a yearly price increase as long as I stay on my existing plan, and it is a win-win. Now, I may be the exception in the IT world, and the fact that I am close enough to a tower to get high through-put has probably influenced my decision.

For those who have a complex setup requiring port forwarding and D-DNS, this setup is probably not for you. I mean, you probably would not run your home network off of a cell phone either. The tech is basically the same. There are limitations with cellular service that just cannot be overcome.

Just realize, that for 90% of T-Mobile Internet customers, this system, as it is, will be more than adequate.