If so, does it have a SIM card installed that has not been activated?
If no, you need to go to an ATT store for a SIM card and have it activated on your account.
You can only add devices on a currently offered plan. If you have an old plan, you can add the tablet as a data only device with its own plan.
If adding it to a Mobile share value plan it is $10 per into line fee and shares your data plan.
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