Weak WiFi until I pop the display open?

I have a similar problem I have experienced on my iPhone 4S. Basically, the WiFi connection is very low unless I am right next to the router, however when I remove the pentalobe screws and pop the display out of the case, the WiFi is perfectly fine. I bought this iPhone 5c used/cracked on eBay, decided to fix the screen myself, and I have not even picked up the motherboard, since I know the WiFi antenna is under it. I have dusted everything with a Q-tip, and I have reset/hard reset my phone several times.

Please help me,

Thanks.