Modem can be the main problem if you dont have signal at all.
But even after flashing the correct rom and modem for your location and you face the problem there could be chances of hardware, very rare though, or could be the area your living at, far away from the city or maybe some area that dosnt fall under a proper network coverage.
could be anything. Just flash the correct firmware rom modem whatever, check that your at an open good place, call and message your friends and check for network problems. Hope this helps.