My guess is that the device has a bad esn. After the original owner lost it they probably didn't pay it off with their cellular provider so the device was blacklisted. The people that sold it to you didn't disable it, the wireless carrier did. Once a device has a bad esn/imei, it's pretty much not possible to make it good again. Of course, the wireless carrier can only stop you from using the cellular data function, nothing else. So you basically now have a wifi iPad