Problem with AT&T connection on iPad 2
Ever since downloading the latest OS on my iPad 2, I have lost my AT&T 3G connection. Have had 2 new SIM cards installed, they work for a short period of time then no service pops up.
iPad 2, iOS 9.3.1
See if there is a carrier update available
Try the steps in here
If you can‘t connect to a cellular network or cellular data - Apple Support
Or if that does not work
Please make a Genius appointment at an Apple Store, or go to another authorized service provider.