Upgraded from a 4s to a shiny new 6. Activation seemed to go fine. I can make calls, and it shows LTE up top, but when I try to use cellular data I get Could Not Activate Cellular Data Network. Called AT&T and did all the usual stuff - reset network settings, rebooted a few times, etc. CS was really nice but ultimately had to put a ticket in. Anyone else seen this? Could it be a bad SIM?