Asus TM-AC1900 is a T-Mobile rebranded router but exactly the same hardware as a RT-AC68U. It is touted as one of the best available routers in the market and is highly rated. However, TM-AC1900 contains a T-Mobile firmware which runs several years behind the official Asus firmware available for the hardware. Thus, it is of interest to "flash" the offical firmware with a newer one.
NOTE: The router works smoothly even without flashing. Don't flash if you don't need to.
Depending on the pre-installed firmware on the CellSpot router, many instructions below may or may not work for everyone. Howevery, they worked for mine.