I noticed that a service is started when the fallback happens but if I kill that service the network also dies. But if you follow the below method it safely starts a new service which kills the one that's responsible for killing the 4G/LTE
1. Upgrade your phone to 13B firmware
2. Root 13B
3. Now download Shortcut Master lite app from play store.
4. Now add these shortcuts to your favorites in the "Shortcut Master lite app". You can search for "LTE" and click on search button
LTE System Selection
5. Now click on "LTEBandSelection" and enable all the bands
6. Save and reboot
7. You now need to enable your data and turn off your wifi. Make sure your phone is in Global mode (in settings >> Mobile networks >> System select)
8. After your phone is rebooted open "Shortcut Master lite" app.
9. Now tap on LTETestMain
10. Select Click on CDMA preference.
Now choose Band preference and select "any"
P_REV_Preference ---> "MOB_P_REV 3
This is important.. EVDO Revision --> Rev A
11. "LTE Connect w/o IMS PDN" must be checked
12. Now go back to your main menu in shortcut master lit app
13. This step is very important. Changing the timeout is important. Tap on LTE System Selection
Except "Scan duration" add a couple zeros to all the other timers. For eg: T_mpsr timer (Min) is 200
The fallback timer will reset itself after reboot. But don't worry the configuration is already updated with the new value. For some reasons the shortcut master lite app is not picking it up.
14. Now finally click on "CDMA preference" in the main menu and make sure it has the value you set in Step 10
At step 10 or 13 your network should go away and your phone should complain about "no network". That's fine. just ignore the message.
Now finally reboot your phone. That's it. You should now have 4G LTE and it won't fallback to 3G or H+
The only problem is when I'm on a call my 4G LTE is dropped to 3G/H+. But after I hang up it goes back to 4G LTE after 12 seconds. I don't know if it's an expected behavior. But I'm trying to see if there is a fix for that.
I tested this with Tmobile. Make sure your APN settings are correct.