FortiAPs, FortiCloud and iOS Devices

I have 4 FortiAPs connected to the Forticloud running on my network and for Windows users, I have no issues but with iOS devices they keep dropping off on iPhones, I can be a couple of feet away and it'll lose connection and go to LTE.  Any thoughts?  I'm running FortiAP 221C and firmware FP221C-v5.4-build0327.