Default [Q] Data only on AT&T for Galaxy Mega

I do not know if this is the right web site, the right forum, or the right sub-forum. Grasping at straws here.

I successfully set up an iPhone 4S as a data-only device on AT&T by obtaining a SIM that was supposedly for an iPad.

iOS7 horrified me, so I switched to a Samsung Galaxy Mega. I swapped the iPhone SIM in and tried to set up an APN to allow my data plan to work with the Mega. It seemed to work at times.

I tried to unlock the iPhone so that I can sell it. I also set up an APN called "broadband" on the Mega, and it seemed to work. Now, that APN is gone (vanished), when I try to create a new APN, it disappears as soon as I save it, and AT&T is asking me to come in and get another SIM.

Not sure what is going on here.

Thanks for whatever advice you can provide.