Had 3rd party fix LCD and now I dont what to iPhone

My iphone 4 was sat on and it cracked the LCD and the back plastic. So i had a cell phone store fix it, which they did, looks good as new. The problem is when I insert a sim card into it, it connects to ATT for a couple minutes then displays no connection, they after about 20 minutes does the same thing. I took it back to cell phone store and tech said he would look at it, he calls back a week later and says everything looks good(antenna ect..) on the inside and it might be a problem with the board.
I just dont know what to do now, I feel like i screwed myself by taking it to a 3rd party to have it fixed. Also, I cant say for sure if connection problem was caused by 3rd party or happened when phone was sat on.
Any suggestions would help cause I just dont know what to do.
iphone 4 iOS 4