I had the same problem , 2 Apple stores said the Antenna was broken , and wanted £200 to replace as it was a few months out of warranty.
I tried resetting network etc etc .. Then tried an update which came up with a 4005 error message , Apple are pathethic , the software updates are rubbish.
I then did a full restore to factory settings .. which crashed , then crashed again even through itunes which i was using . Then 3rd time lucky it worked and the sgnal was back.. phone now working .
So Apple store advice of the phone being needing replaced couls have cost me £200 .. disgraceful service .