Isn't the provisioning going to be for the SIM, not the tablet? In which case it's a matter for ATT's systems, not your device, and so fiddling with the tablet software won't change anything. When you try to connect ATT will receive the IMSI (they have to know which SIM you are using to associate it to an account) and the IMEI (which will tell them you are using it in a tablet). So if the reason you don't get data is because their systems say "phone SIM in data-only device, uh-uh" then there's nothing you can do (I'm not discussing changing IMEI since while I don't know the law in your country it's specifically illegal in many countries, including my own).
Of course if ATT don't impose some additional silly restrictions in the phone's software there's nothing to stop you turning on the hotspot and using wireless tethering to connect the tablet. That's what I do with my own tablet or laptop, but I gather that some carriers see the ability to tether wirelessly as something they want to charge extra for.