I was a TMO customer forever but a couple thing happened.
1. I moved to Maine and there is ZERO TMO data service here
2. I got tired of waiting for TMO to give me KitKat
To correct problem #1, Knowing that our T999 works just fine on AT&T, I decided to change networks. I refuse to become an actual AT&T customer so I went to Straight Talk for a SIM that uses AT&T as the network. I got the SIM and pay 45 bucks a month for unlimited Voice, unlimited Text and 5 GB of Data. I reset the APN and everything is working Marvelously.
To correct problem #2, Although I flashed a ROM a Day back in my Vibrant days, did a bunch of theming and made a few ports myself back then I haven't done poop since I got the S3 a when it came out a couple years ago. Time to change that.
With some direction from joe3681
I decided to bite the bullet and now I'm runningAT&T TW KK NE4 4.4.2 on my T999 on AT&T via Straight Talk.
How to do (you know the routine):
1. You obviouly are rooted with a custom recovery (I use TWRP)
2. Backup, backup backup
3. Grap the ROM, KERNEL, MODEM (from links below) and put the on your SDCard or Ext SDCard
4. go into recovery and wipe, factory reset etc.
5. flash the ROM, then KERNEL, then MODEM
6. Fix permissions
7. reboot and let things settle out.
8. login to Google, restore all your apps and such. ( I let Google Play reinstall them vs Titanium) (don't restore app data please)
9. Viola, TW KK 4.4.2
ROM: I747UCUFNE4_AT&T 4.4.2 (I used Stock Odex)
KERNAL: [KERNEL][AT&T][AOSP/TW][4.1/4.2/4.3/4.4][07/11/2014] KT747 - ND8 - KTweaker (TW KiktKat 4.4)
[Baseband] ALL T999, T999L, T999V, T999N Modems! w/ MAP and Data Tweaks!
Everything so far that I've tested is working just fine. You do get all the AT&T bloat (but no TMO bloat), but you can delete it later or from the ROM before you flash if you know how. Connected to HSPA+ so data is fine.
Here's the APN settings for Straight Talk AT&T I'm using:
Name: Straight Talk
Proxy: <Leave in Blank>
Username: <Leave in Blank>
Password: <Leave in Blank>
Server: <Leave in Blank>
MMS proxy: mms3.tracfone.com
MMS port: 80
Authentication type: <Leave in Blank>
APN type: default,mms,supl,hipri