Cannot connect to App Store via cellular
I cannot connect to the App Store using cellular data. I have confirmed that cellular is enabled on the phone and for the App Store, my iOS is up-to-date, and my time/time zone are correct. I have signed out and back in. I have powered down and back up again. I can connect via wifi, but I still can't connect via cellular. Could this be a service provider issue? I have a Verizon iPhone 6 which is able to connect, but the issue is with an AT&T iPhone 6.
Windows XP