Now, you can just use the router out of the box, and you don't need to be a T-Mobile customer. As far as I know it just has some built in QOS stuff to prioritize the voice traffic - the idea being that you can make calls from your phone using the WiFi via the router when you may have less than great cell service.
However if you are willing to do some work with some risk involved (warranty voiding, brick the router risk), you can turn this router back into an AC-68U and load any available firmware. The way it works is that you first downgrade the TM-1900 to an earlier version of the T-Mobile specific firmware (376.1703) - this is done since the later firmware the router ships with does not support telnet, which is required for the modification. Once you have downgraded and enabled telnet, you need to modify the CFE (the BIOS for the router). Changing the CFE will change what the router "thinks" it is- that is, it will change it from the T-Mobile "TM-1900" to an Asus "RT-AC68U". Helpfully there are versions of the CFE from Asus RT-AC68U's out there which you just need to modify with a hex editor to change a couple of key values from the TM-1900's original CFE to the CFE from the RT-AC68U (whcih would be mac addresses and a "secret key"). You would get the original values from the router by telnetting to the TM-1900 and running a command to copy the CFE to a USB disk connected to the router - that file would then be used with the hex editor to get the original values for the 3 MAC addresses and the "secret key" which would then be used with the CFE for the RT-AC68U (several versions available in the thread linked below). After tripple checking the modified RT-AC68U (checking that it has the correct secret key and MAC addresses from the original TM-1900 CFE copied from the router), you would then overwrite the router's original CFE, thus changing it into an RT-AC68U. After that you can just upload the merlin firmware or whatever, however I ran into a problem...
My router was originally on the TM-1900 CFE version 22.214.171.124 which seems to be locked down since when I tried to overwrite it with an Asus RT-AC68U version (tried 126.96.36.199), the CFE version the router reported did not change (was still 188.8.131.52 when running nvram get bl_version <Enter>). My workaround was to flash with the earlier TM-1900 CFE version 184.108.40.206 which actually will let you flash some custom firmwares anyway - so I could have stopped there, but once on 220.127.116.11, I re-flashed with the Asus RT-AC68U specific version and it took - I think I used the 18.104.22.168 version. You will know when it is on the right version since the router model will change when you log in - it will go from TM-1900 to RT-AC68U.
Now my router is functionally identical to an RT-AC68U for about 1/2 the price, however the gotcha is that this will presumably void the warranty. I am testing the merlin firmware on it now and it seems to work as expected. This router is suffecient for VPN (I have read that max throughput with OpenVPN is around 20Mbps), and it will also run a VPN server. Running a VPN server seems a better idea to me than opening ports on the router for things that only you need to access on the LAN - I use it for a few security cameras so I can check them from a phone (OpenVPN android client + IP Cam Viewer). Running a VPN client from the phone also secures data when connected to open WiFi.
As an aside, I already voided my warranty when I decided to repair a broken USB 2.0 port on the new router rather than just send it back - I was halfway through the upgrade and the part was a couple bucks. First time I have seen a USB port get munched, but it was probably partly broken out of the box. In hindsight I should have sent it back since the solder on this thing was very difficult to work with even after clipping the bulk of the USB jack from the board. It just would not heat up even with my highest wattage iron (probably needed to pre-heat the board and use a hot air station which I don't have). It took me about an hour just to desolder the old USB jack, but it does work now, and allowed me to complete the CFE mod and firmware upgrade. The case is held together by 2 screws behind the sticker on the center back of the case, as well as some clips around the edges. This USB problem is a fluke, I have never seen this problem on the other AC-68U's I have owned or set up.