Success! Flashing the 4.2.2 modem seems to have done the trick. Wish I knew about this the first time, would have saved me an entire day of messing around with flashing lol!
Basically what I did:
0. Did not insert SIM card yet. On microSD card: aboot.img (aboot4.2.2), modem.img (modem4.2.2), CleanRom's ZIP, Stumproot.apk
1. Flash the 24A kdz with LG Flash Tool 2014
2. Boot device, use File Manager to install Stump Root.
3. Root Stump Root, brute force mode, reboot when done.
(3.5. I connected to wifi and installed SuperSU from the market, but I don't think it was necessary)
4. Enabled USB debugging, copied aboot.img to /sdcard/, followed TWRP install instructions.
5. Boot into recovery, flash v126.96.36.199 TWRP update, reboot into recovery.
(5.5. Again, not sure if it was required, but I wanted to get rid of any dead files from the stock install, so I used TWRP to format the device, then rebooted back into recovery.)
6. Installed CleanRom and copied modem.img to /sdcard/ via TWRP.
7. Booted into CleanRom, enabled USB debugging, flashed modem.img via these instructions: http://forum.xda-developers.com/show...96&postcount=5
8. Power off, stuck in SIM card, powered on.
9. Set up TMO APN, connected fine, visiting ipchicken.com showed a TMO IP/hostname.
10. Powered off, waited 10 minutes, powered on again, still connecting!
Hopefully this'll be the end of my troubles with this tablet. I'll update this post if something goes wrong in the next few days.
My Tablet is now showing 4G LTE but it does not connect for some reason. It shows my number etc but Mobile networks state: Disconnected
On the T-Mobile site it still shows my old tablet instead of the lg g pad 8.3
Any ideas? Also what is your apn? I tried a few I found on here but nothing seemed to make a difference. I have a Tablet data plan from T-Mobile.