Had the same problem after updating the software twice recently. I tried everything from resetting network connections, updating/syncing with itunes, switching off airplane mode, factory reset, etc. etc. I put it in the feezer and this fixed the problem for only a few hours. I then tried the hairdryer and it only worked for a few hours too.
But then I read that you need to get the iphone really hot and focus on the top right section of the front of the phone with the hairdryer. I used the hairdryer for 3-4 minutes and the phone was quite hot. I read the hotter the better but I didnt want to risk too much heat. But it was hot after this. I immediately switched on the iphone and my wifi signal was better then ever. After a few weeks I then received another ios update and decided to chance the update a few days ago. I immediately lost the wifi again but repeated the hairdryer treatment and once again wifi back and working really well. So, try it and see. Definitely worked for me. Lingmell.