I installed stock rom using ODIN:
It booted now. NB1, 4.4.2, MDF.
Now, can I use towelroot for rooting?
I am not an expert and although I read so many pages, I ended up the problem: System software not authorized by ATT has been found on your phone.
You can call me stupid, it is fine, but the process is very complicated for normal users.
I was on 4.2.2. and MDL. I installed safestrap and backed up. I flashed this ROM:
I337UCUFNC1 100% Stock Rooted ROM
Because I wasn't on MK2 (actually even earlier version), I didn't do downgrading steps.
Now, once power up, the phone stays "SAMSUNG Custom" screen.
When I tried to put it to recovery using "volume up+power+home" it shows:"System software not authorized by ATT has been found on your phone."
When I try "volume down+power+home", I can enter odin.
Any help will be much appreciated.