To the point, using ODIN, I flashed TWRP onto my T999 (after heimdall kept failing for me), and I performed a back up and then proceeded to flash CM 12.1 nightly onto my phone. The nightly worked great other than giving me an error of "SIM card missing" in the setup screens. This of course is a deal breaker for me as this is the phone I wish to use daily. I did my research and found that for many people this was the case, however the network would still work despite the phone's status saying otherwise, though my phone would neither receive nor send texts/calls let alone find any network operators. (I also tried different releases)
I then tried to flash multiple CM 11 official releases but kept receiving "assert failed getprop( ro.bootloader)..... " I even tried the ol' "delete that line of the script" troubleshooting, however the installation would then abort just the same without stating the error.
After both of these failed, I tried recovering my previous back up however it failed every time.
I currently am running PAC ROM's 5.1.1 which yields the same issue as stated above concerning the nightlies.
I have also tried updating my modem as I found under troubleshooting that fixed what seemed to be the same issue for others, yet it never worked for me.
Any advice would be great. I'm sure you'll need more info, so let me know what you need.