wifi

i just bought 2 iphone4 two days ago and when i got home only one of them connects to wifi. so thast night i called att and they tried to help for over an hour but couldnt help they told me to bring it back to the store i went the next morning and they told me to call enchanced support so i did and talk to them trying everything but didnt work so they said go back to store and exchange it but they wouldnt do it told me to go to apple so im trying to see what i can do to try and fix this problem. can anyone help me