SGH-T999 Custom ROM with working 4G

Greetings,

I really don't want this to be a repeat post, so I have spent many days now searching for this answer, not just here on XDA if I was honest, I have links to prove it ;)
The aim is to give a little new life to my old phone, and get off the 4.1 build it is on.

With that said, in summary, I have tried a few different custom ROMs; Cyanogenmod 11 - 13; stable to nightly. PAC MAN ROM for Lollipop, some other roms for KitKat I found else where, but in all these I have not been able to get 4G to work. I understand about updating the Modem, and got the latest version from DocHoliday777's post for the Galaxy S3, in point of fact I tried every modem version for 4.3+ on there :D
But for all this, and it's been time consuming I never managed to actually get 4G to work as stated :(
I tried playing with settings in Test Mode (*#*#4636#*#*), changed preferred settings, added APN's, rebooting during certain phases of the moon :)... but somehow it all avails me diddly.

Finally I have genuinely tried searching for info on if this just isn't going to happen and that's how it is, but somehow I found lots of posts about other models or networks that made it work more than something saying this won't work (putting aside the info on CM 11 that states it's a known issue and won't work on some build I forget).

Which leads me to this post: I wondered if anyone could point me in either the direction of a 4.4+ custom ROM or other ROM, compatible for SGH-T999 (T-Mobile Galaxy S3), that has working 4G. Or how I can get 4G to work on one of the ROMs I have already tried, kind of partial to a nice CM but I am willing to venture.

Also happy to just hear other input about giving new life to the phone so any feedback welcome :)

Thanks for your time!

NOTES
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Using TWRP 3.x for flashing
Phone is rooted
not experienced at this but willing to read :)

Installing CM13 on SGH-T999

My phone is a 4.3 official Samsung and is properly rooted. I have downloaded CM13 and Gapps onto my external SD card and went into CWM recovery mode. I firstly backed up everything (thankfully) and performed a wipe data/factory reset followed by installing CM13. However when I run CM13 almost immediately an error pops up saying "this package is not for device: this device is d2spr" along those lines. Any help would be appreciated; thanks!
BTW forgot to mention I completely removed Knox from my phone so that I could root it.

How to unlock SGH-T999 if using 4.4.4 custom ROM

THESE STEP BY STEP INSTRUCTIONS ARE FOR THE SGH-T999 MODEL

I few days ago I had to switch from T-Mobile to AT&T. Unfortunately the unlock PIN that T-Mobile gave me did not unlock my rooted SGH-T999 with the
[ROM][OFFICIAL][AOSP][KK][05/09] **C-RoM** -***4.4.4***- -v7.1! - XDA Forum installed.

After doing some reading in the forums, it seemed that downgrading a phone from >=4.3 down to 4.1.1 in order to use the free unlock method, would result in a hard bricked phone. If I remember correctly I also read that downgrading just to 4.3 would also result in a hard brick. I decided to risk it and go for the 4.3 downgrade.
This is what I had to do in order to unlock my phone.

Download the pre-rooted 4.3 stock ROM named T999_UVUENC2 from here [Firmware/OTA's] [root66] SGH-T999(All Variants!) Official & Root66 FW & OTA Updates!

Call your original carrier and ask them for the unlock code (you'll need the main account holder's name, last 4 digits of their social security number, the phone number of the line being switched over, and possibly the IMEI of the phone being switched over).

Make sure your phone has Developer options/USB debugging enabled.

Make a recovery backup of your phone (I suggest saving it in the SD card. I used TWRP). Turn off the phone.

Start your phone by pressing and holding the POWER + HOME + VOLUME DOWN keys until the screen with the yellow triangle with a black exclamation point appears.

Press the VOLUME UP key.

Plug your phone to the computer you intend to use Odin from.

Download Odin 3.09 (for some reason the 3.07 linked here: [Firmware/OTA's] [root66] SGH-T999(All Variants!) Official & Root66 FW & OTA Updates! would not work for me).

Start Odin 3.09 with admin privileges.

Make sure only "Auto Reboot" and "F. Reset Time" boxes are CHECKED/TICKED.

Click on "AP"

Select the pre-rooted stock ROM you downloaded earlier.

Click "Start"

Once the top left box in the Odin window is Green and says "PASS!" you might find your phone stuck in a boot loop (in my case my phone kept showing the Samsung logo, while periodically flashing the option/back buttons, and vibrating the phone briefly), pull and re-insert the battery.

Press the HOME + POWER + VOLUME UP buttons (when you feel a vibration let go of the POWER button).

<<< Run the wipe/factory reset AND the wipe cache options. >>>

Turn off the phone, insert your new SIM card, and finally turn on the phone.

Your phone should boot properly and show you the welcome screen. The network unlock PIN screen may appear right after the welcome screen appears or after you complete the initial configuration. Enter the network unlock PIN. It should give you a "Successful" message.

Restore the backup you made earlier by first installing the recovery application through Odin (I use TWRP. Follow the same steps for installing the stock ROM), turn off the phone, enter recovery mode by pressing POWER + HOME + VOLUME UP (let go of VOLUME UP when you feel a vibration), find and run the restore image you made earlier.


I hope someone finds this useful.