Without much context, I assume that you have H500 solution running on ZD because this is a medium / large space like a campus area, dormitory or smaller hotel. The issue you're having is 'mostly' driven by the selection the client device makes. I don't think iPads have a built-in roaming aggressiveness feature you can adjust like the NIC in most PC's, which can certainly help from the client side.
1) ideally you want to find out why they are roaming - there is going to be some kind of interference behind the choices, or the APs are installed with a far too strong -dB overlap and it may just find the other loud AP to have a clearer channel at the time.
2) dial down your 2.4 so that its usable range is about the same as the 5GHz, drop it by ~3dB and then check again for dead zones you probably created. Then see what you might need to do re: moving some AP or adding some antennae that would give both your 2.4 and 5 ranges additional range, dialing back the 2.4 as needed.
Lastly, if you are using the H500 range because this is meant to be an AP-per-room type situation, then you may just need to dial down the signal altogether so that it becomes solely usable within that room, or parts of that room.