My C6603 has the same problem,
I have tried almost everything to fix it.
Have you backed up the TA partition,if so then try restoring it.
But you may need a new motherboard.
So,if you find any fix for this,plz pm me about it!
Sony Xperia Z C6603 Black -> Broken BASEBAND (Detected as Unknown) . Can't get any signal!
CarbonROM KitKat 4.4.4 + Lightning Kernel
OC'd to 1.83GHz-CPU and 533MHz-GPU