Here is a video tutorial to repair the no wifi issue on an iphone, after it has been repaired.
Apparently, when the iphone motherboard has been removed, there is a small spring that acts as the antenna for the wifi card. When the motherboard is pulled from the iphone, that small spring goes off and it falls off. What you need to do is find that metal piece and put it back where it was.
Here is the video explaining everything: