Fix low signal on iPhone 5?

Hello :-),

I am having an issue with a phone that I just fixed. It was a phone a bought on eBay that had been beat up so I didn't know the function of the phone. After I hanged the housing and screen everything works perfectly on the phone. The only issue I can't figure out is the low signal strength. I tested the SIM card I my personal phone and the repaired phone and it is not the SIM card or the wireless provider. The repaired phone does get cellular signal but it is very low, Lowe then usual when compared to the same phone at the same place with the same wireless carrier. Any ideas?

Thank you.