IPhone 5 won't work on Wi-Fi?

So my mom's iPhone won't work on our Wi-Fi but my iPhone does. So do my other devices. I turn off the Wi-Fi on the other devices and hers still won't work. I reset it won't work. I reset the Wi-Fi and still nothing. Can we replace it? Do i go to the apple store to replace it or at&t store? We got the phone from at&t. Mom has a contract with AT&T btw so yeah

IPhone 5 works on Wi-Fi, you should go to AT & T and exchange the phone.