It must not be a Apple setting problem, it's mostly a problem on the users side. There are two advices for the iPhone and iPad users:
1) DO NOT USE ANY OTHER APPLICATION except QMix/UC Surface, as some applications continuously demand an internet connection. This causes conflicts forcing QMix and UC Surface to disconnect.
2) NEVER LET YOUR iDEVICE GO SLEEP (http://answers.presonus.com/6416/establish-iphone-connection-mixing-board-after-goes-inactive).
And there are two general advices: Place the router "in sight" of the iDevices and use the "dual band" feature (2.4 and 5 GHz).
Hope this helps.