Install Samsung kies 3 from here
Run kies then go to Tool > Firmware Upgrade and Installation
Follow the on screen instructions, this should completely reset the phone to a working condition.
Point is, I can use stock 4.3 rom without any problems, but my issue is that I can no longer change to non-TW roms, esp KK.
PS: I can get stock 4.3 without any problems. The signal problem occurs in non-TW roms only. I wanted signal on a 4.4-based rom.