Just set the iPhone to tether mode, whatever it's equivalent is in Apple. The tablet connects via wifi as if the phone is a pocket wifi modem. I have my Samsung Android mobile set up with a widget that I can turn on tethering in one press. Either wifi or USB. Works really well, I tether my tablet, other phones or laptop etc, anything with wifi or USB.
The USB tether is good as the phone is charging whilst acting as modem/router.