I'm running Mahdi 6-28 build, V6 patch, 4.4.4 PA mini gapps, 24A baseband
After several hours of searching and troubleshooting I found a fix. Trying to make APN entries didn't lead anywhere, and editing the apn-conf.xml also did nothing. I had flashed 4 or 5 different 4.4.4 roms (some stock, some AOSP) and still nothing, but for whatever reason, doing a wipe and flashing XDABBEB's VS98024A v1.2.1 ROM (can't post link sorry) fixed my mobile data. I then did a system/cache/dalvik/data wipe and reinstalled Mahdi 6-28 and boom I have Mahdi with working LTE. Sorry for the spam I guess, but maybe this will help anyone having similar issues.
Ran into the same exact issue on a friend's phone. Going to try this out. Regardless if it works, thanks for the post!
UPDATE: It worked. The 24a bootstack includes the new modem and properly formats relevant partitions. I flashed XDABBEB's ROM just to replicate your exact procedure but this shouldn't be necessary. The bootstack is all that is needed when coming from a 12b rom.
Return it to stock, let it take the KK OTA, root it, autorec for recovery, flash your 24A based rom of choice, that always works for me.
I had Carbon running fine on mine til I got bit by the black line issue. I haven't put AOSP on the replacement, probably will soon..