[Galaxy S 5 General] FNetV1:
Here has been my experiences with this experiment so far:

My device: SM-G900P that was used with Sprint before but now its been used on AT&T (unlocked it) in 3.5G (h) mode.

My baseband is: G900PVPU3BOK4

Here's what I have done so far:

Running: ResurrectionRemix Marshmallow (6.0.1) I installed Terminal Emulator and entered the following commands:

su

setprop sys.usb.config diag,acm,adb

getprop sys.usb.config

Then I enabled USB Debugging mode and told the ROM that I would like to perpetually trust my own computer.

I opened up EFS Professional (version 2.1.80.0), clicked on EFS Professional (the first tab), then clicked on "Samsung Tools", then on "Current USB Settings" I changed it to "DIAG + MODEM + ADB", a box appeared that said in green letters "connected" and the change was successful.

Next, I opened up QXDM, went to "OPTIONS" and "COMMUNICATIONS", saw my device listed there as "COM10", it was the one that identified as "MSMxxxx" (where "xxxx" is four numbers that I don't fully remember right now) under the "Phone" column.

In the bottom where it said "Target Port" it said "disconnected", I changed it to COM10", then pressed OK. After doing this QXDM connected to the phone successfully.

On "View" I changed it to "NVBrowser"

Then on Category filter I changed it to LTE

After doing this I was able to see the "06828" and "06829" entries.

I clicked on 06828 and clicked on "READ" to read existing values. It read them fine.

I entered 0xFFFFFFF3FFF on input, after pressing enter the value showed "17592185995263", I clicked on WRITE to write it, then I clicked on the second line, did the same and then clicked on "WRITE"

After that I proceeded to the next one: 06829

I did exactly the same thing. On the first line enter: 0xFFFFFFF3FFF which gave: 17592185995263 and then clicked on WRITE and then on the second field entered 0xFFFFFFF3FFF which gave: 17592185995263 and the clicked on WRITE.

After doing that I verifed that the values were indeed written, I "READ" then all and QXDM did print out the new values on both fields just fine.

After all that I restarted the phone, I noticed I had no signal, mobile connections were set to "LTE preferred", I typed *#*#4636#*#*, phone testing showed up, I tap on Phone Information, there I saw that the connection mode were changed to: "LTE/CDMA", that's why it wasn't connecting, I changed it to LTE/GSM, after doing this the phone connected but only in HSPA+.

Furthermore: following the connection technique of the first poster I searched for networks, it scanned available networks and the following appears on the fields:

AT&T

T-Mobile

Sprint

and two other weird numbers (6 numbers) (don't know what carrier those supposed to be - but I think one of them could be verizon).

I tap on "AT&T" and then said "Registered successfully", but still no LTE showing up,

Then I tap on "T-Mobile" and it took literally forever for it to error out, after that I tap on AT&T and once again it said "Registered on Network", but still no LTE showing up.

Next, I went back to *#*#4636#*#* and turned OFF the radio and then back ON, I tap on "GSM auto (prl)" and then I tap on "LTE/GSM audo (PRL)" but still no LTE connection, the phone only connected in HSPA+ mode all this time.

Further wanting to explore the issue I installed the app: "Network Signal Guru"

On this app I went to "Lock bands" and saw a whole large list of LTE bands as "Available Bands", this prooves that my QXDM maneuver did succeeded in recording these values onto the phone. However then I "RAT" locked to LTE and when to explore what is going on there, the LTE radio, despite on having all these 15+ LTE bands as available, only camps in the following three bands: B25, B26 and B41 which are Sprint's LTE bands.

No matter what I did I was not able to get my phone to make an LTE connection on band 4 which is AT&T's LTE band or any of the other three AT&T LTE bands, there was no activity on any other bands shown on the Network Signal Guru app.

Thinking that the problem might have been that I am using the Marshmallow version of android to do this and that somehow Google might have patched the kernel by this version of android, I went ahead and installed "MultiROM" on my phone, I flahsed the MultiROM version of TWRP and in a MultiROM slot I installed the following ROM:

Cyanogenmod 11.1 (the Kitkat version of Cyanogenmod for Unified KLTE for the G900(x)

Then I booted the phone onto this ROM by selecting it on the boot menu, then after changing ROOT functions on developer mode to "Apps + ADB" the third option, I repeated everything I did on my primary 6.0.1 ROM, step by step exactly as I said it above, and the results where exactly the same, after I installed Network Signal Guru onto this ROM to see what was going on, the LTE radio was camping only onto these three Sprint LTE bands.

I mean, this is the oldest AOSP ROM available for this phone, I dont know what else to do.

What I will do next will be to MultoROM onto Cyanogenmod 12.1 and see what happens there. Will try that in a few more minutes or hours since I have to step out now.

Just wanted to mention I did all that, tried it on Android AOSP 6.0.1 a Cyanogenmod fork called ResurrectionRemix, values were saved just fine on QXDM, no dice

Then I retried it on the oldest possible version of Cyanogenmod 11.1 which is android Kitkat AOSP and still no dice

I will soon retry it on Cyanogenmod 12.1 to see what happens there as that is one version mentioned here where I think a poster said they had a or some success enabling LTE bands.

Any ideans? What else can I try, am I on the wrong baseband or something? I am aware that I cannot downgrade my baseband because I am on one of those versions that doens't allow you to downgrade the bootloader, however I might be able to downgrade the modem by flashing the modem (dont know about this) but I doubt this could be a problem because a poster posted a screenshow showing a banseband version more recent that mines (mines were released in year 2015 and his was released on year 2016) and he posted a screenshot proof that he was able to unlock the LTE bands. so unless there is something wrong with my baseband or I forgot to do any critical steps, I am at a loss here.