I purchased a "new" unlocked Samsung Galaxy S4 Active SGH-I537 phone on E-Bay. Everything works fine except when I use the phone with my local carrier SIM card. When I insert the SIM card and turn on my phone, the carrier name appears and I get full bars. I can make a call or two, but then the carrier name is gone and no bars. At times I get the message: "Mobile network not available". I cant make any more calls and if I try to the whole phone freezes and I have to do a hard reset. Ive tested the SIM card on my old phone and it works perfectly. I have checked and the frequencies are the same for my local carrier and AT&T's.

I would appreciate any help on this matter.
Thanks.