At the suggestion of others on a related thread, I have found that switching to wireless N - 5.0 GHz mode gets my iPhone working again. Signal strength goes back to "Excellent". No distance problems, etc. All other modes still create the problem. The MAC address filtering and choice of password security do not impact at all. Of course, I can't stay in that mode because then other devices I have do not work.
APPLE - PLEASE TAKE NOTICE AND GIVE SOME ACKNOWLEDGEMENT!
Maybe this issue is your WLAN signal be weaked between your iPhone or iPad, you can try to replace your WLAN or to paste "Perfecting Signals" on your WLAN Box for improve its quality of the WLAN signal.
More detail info to see below link: