[Galaxy S 5 General] najoor:
This thread has been dormant for a while but I since I spent days trying to make this work I figured I should post my findings here .

I am fairly convinced now that changing NV items only allows ATT LTE B2 to be used through Sprint LTE B25 modem that is already implemented in the phone (both B2 and 25 use 1900MHz). It will not add LTE capabilities for T-Mobile for example (which uses B4 and B12). It also seems to add LTE B3 (not used in the US.) I tried to use a US T Mobile SIM card in Germany and it connected to roaming LTE via band 3. I also tried a German Vodafone Sim card which would ordinary work on LTE band 20 but in this phone it switched to LTE band 3.

It took me a while to get the whole thing with EFS Professional and QPST and QXDM up and running (it is not as simple as it may seem from the OP. There are intricate steps involved and if you miss them things just will not work). I managed to successfully change the NV items responsible for LTE and I made sure they stuck. I used stock rooted Lollipop OG1 ROM to apply those changes (because people say it is not possible to get into DIAG mode on Marshmallow to do the NV edits) but on OG1 I could get absolutely no LTE for ATT. Then, after applying the NV edits, I flashed the latest ROM (QD1 as of today) and the following happened.

First of all, You can set the network mode to "Automatic" via the settings menu or to "LTE/GMS/CDMA auto (PRL)" via Device Info in secret code 4636 using the phoneinfo app. The phone will boot up to ATT LTE (if of course the proper APN is set). Then, if you use the 197328640 secret code and press [1] Basic Information, you will see something like: "LTE RRC: CONN BAND: 25 BW:20MHz" and "MMC-MNC: 310-410" which tells you it is connected to ATT LTE but using the modem capabilities on Sprint band 25. I did a speed test and I got ~20MHz UL/DL which tells me I am not on 3G. However, as the OP mentioned, this connection may last for a while but will switch to 3G, for example when you dial a number, after which you need to toggle the airplane mode or reboot to get LTE back. The only way I've seen this connection go back to LTE by itself is when I'm driving around and the phone keeps changing cell towers and eventually latches on to LTE. So that is how you get band 2 (or really, recycle band 25). If you force it "LTE only)" via the 4636 secret code option, it seems to stay on LTE but phone calls won't work (SMS and MMS still work.)

Secondly: before you change the NV items, if you enter the service mode and make your way into the menus all the way to the Band Selection menu, you will see this:

[1] Automatic (*)
[2] WCDMA Band Preference
[3] GSM Band Preference
[4] LTE Band Preference
...

Which implies that the band selection is Automatic. Now if you go into item [4] you see this:

[1] LTE B2
[2] LTE B3
[3] LTE B4
[4] LTE B5
[5] LTE B7
[6] LTE B17
[7] LTE B20
[8] LTE ALL

If you press on any of these bands it gives you a message as if it is selected but if you return to this menu you don't see a (*) next to the supposedly selected band, which means it is not selected.

However, after the NV item change, the ONLY band in this menu that can be successfully selected is B3:

[1] LTE B2
[2] LTE B3(*)
[3] LTE B4
[4] LTE B5
[5] LTE B7
[6] LTE B17
[7] LTE B20
[8] LTE ALL

Selecting other bands does nothing, which makes me believe that the only additional band after the NV edit is just B3.

Also if you have your phone's MSL you can dial ##3282# (##DATA#), press Edit, Enter MSL, press LTE, press LTE roaming enabled, and set it to ON. I don't know if it affects anything. It doesn't seem to stick and goes back to OFF once you leave the menu.