Mobile network not available on unlocked Active 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 can’t make any more calls and if I try to the whole phone freezes and I have to do a hard reset. 

When I first turn it on and don't try to use the phone app, I can go into Mobile Network and see the list of my local carrier in Network operators. If I try again to go into Mobile Networks, the network bars go to zero and get the message "Insert SIM card to access network services."

I’ve 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&amp;T's.

I would appreciate any help on this matter.
Thanks.
First, make sure the APN settings on your phone are correct for your carrier. Next, try cleaning the SIM card's and phone's contacts. You should also try a different SIM to rule out a broken card (also try your current SIM in a different phone.)

If the problem persists - then it's most likely a hardware issue with the phone's SIM card connector.  You may want to get in touch with Samsung to send the phone in for service.

The SIM card contacts on the phones are notoriously fragile, and can be easily damaged by using an adapter (Micro-to-Nano for example) that is either defective or simply improperly manufactured. If the adapter is not an exact fit, the contacts can get "caught" in the gap and bend or even break off during removal.  It's possible this is what happened to the previous owner of your Galaxy S4.

This type of damage is usually not covered by manufacturer's warranty, but it won't hurt to try anyway.  Worst case - there are SIM housing kits available for sale for the Galaxy S4, and ste-by-step replacement instructions on YouTube.  It's fairly easy to replace this module on your own.