have you tried set correct date and time, then turn on airplane mode in settings then go to GENERAL THEN SCROLL TO RESET AND RESET NETWORK SETTINGS, WAIT FOR PHONE TO DO ITS RESET, THEN GO TO SETTINGS AND TURN AIRPLANE MODE ON AND THEN OFF AGAIN and watchfor search to find network. This has worked for me before. I would double check aenna grounding has been put back correctly under top screw of battery terminal.
good luck bob