[LG G Flex 2] csrow:
This guide is for converting AT&T H950 to be used on T-Mobile network. It's not for novice as some manual edits are required. As usual there is a chance you may brick your device. YMMV.

These are the steps I've taken to go from SIM locked stock AT&T H950 v11z to make it work with T-Mobile. It should work the same way for v11x.

1) If the phone is SIM locked, get a SIM unlock code (either by requesting from AT&T or purchasing an unlock code online). The online unlock codes cost around $10-$13 range. This page is where you can request an unlock code from AT&T even if you are not an AT&T customer.

2) Root the phone using the proper rooted system image. You can do it using the 'easytool' method or by manually using the 'dd' command method. BE SURE TO DO A SYSTEM.IMG SAVE FIRST. This is so that if you want to go back to the original form, you have a way to go back. If AT&T ever updates to MM, you will have to be in the stock unrooted state for the OTA will work.

***Make sure you understand how to root before attempting.*** ***Save your original image***

3) If AT&T spying on you bothers you, delete following two files to block Carrier IQ. You will need a file explorer which can access the root area. I would recommend 'Root Explorer'

system/lib/libiq_client.so
system/lib/libiq_service.so

You can check the result by running the 'Carrier IQ Red Pill' app. It should report 'unknown'.

4) Using an editor (I would recommend 'QuickEdit') Change following two lines in the build.prop file. The 'build.prop' file is very dangerous file to mess up. Be careful. Make a back up copy first and after the edit, make sure the permissions are set the same as the original. If you are not sure what that means, stop here and do some research.

system/build.prop line 292 change 'ATT' to 'TMO'.

system/build.prop line 392 change 'unknown' to 'TMO'.

5) If the AT&T animation at boot up bothers you, change bootanimation.zip to something else. I went with the just generic LG animation but there are many other cool choices online. Be sure to set the permission the same as the original file.

system/media/bootanimation.zip

6) Reboot and check AT&T animation is gone (if you changed to boot animation).

Go to Settings/Tethering & Networks/Mobile network/Access point names and add a new T-mobile APN.

Name: T-Mobile US LTE
APN: fast.t-mobile.com
Proxy: <Not set>
Port: <Not set>
Username: <Not set>
Password: <Not set>
Server: <Not set>
MMSC: http://mms.msg.eng.t-mobile.com/mms/wapenc
MMS proxy: <Not set>
MMS port: <Not set>
MCC: 310
MNC: 260
Authentication type: <Not set>
APN type: <Not set>
APN protocol: IPv4/IPv6

After adding the APN, select it as active. Reboot again.

7) Download and run 'shortcut Master(lite)'

Search for 'ltebands'. Run the hidden menu.

Click on band 2 and band 4. It should come back with success. (I don't know how to enable band 12. It comes back as a failure).

8) Reboot and download and run 'LTE Discovery' app to make sure LTE band is working (and not just UMTS or H+). It should show either band 2 or 4 LTE. Turn off wifi and make sure mobile data is running. Do a speed test to see if you are getting an LTE level speed.

9) Download and run 'Hotspot Control' app to control hotspot.

10) Optional - Use TB to freeze AT&T bloatware. DO NOT DELETE. Just freeze.

That's it. You should have a fully functional T-Mobile phone now. Let me know if it works for you.