Cellular connection kicks in when the WiFi is weak, that's a nice feature in iOS 9: WiFi Assist.

I have on question, when WiFi Assist is ON. Does it cause me cellular data in the following scenario:

  • WiFi Assist is ON
  • Settings > App and iTunes Stores > Use Mobile Data is OFF
  • Now, WiFi is good
  • I am downloading a huge app
  • ... so far so good
  • suddenly WiFi goes weak ..

Does AppStore continues to download and consumes my cellular data, or just stops?

  • I dont see such option in IOS9 (iphone6), region Latvia. Maybe it is US-specific? – user147072 Sep 17 '15 at 4:09
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    Go to Settings > Cellular and scroll all the way to the bottom, even past the long list of apps. – user24601 Sep 17 '15 at 4:13
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    The option is inside Settings > Mobile Data > ... scroll to bottom – ohho Sep 17 '15 at 6:40
  • I have screamed through my cellular data cap and suspect that it uses cellular data when wifi goes weak even if cellular data is switched off, have not done testing to confirm this. I have switched wifi assist and cellular data of in meantime. – Joop Sep 25 '15 at 13:49
  • I am on the phone with Apple (15 min) and they cannot tell me what the threshold is in Mbs that engages Wifi assist. They say it is different in different places? Inside houses and offices it is most likely constant. What? Mbs are mbs. What a windfall for the carriers who I find (AT&T) not forgiving. Thanks Apple! – user149032 Sep 28 '15 at 19:58

In your case the off switch for mobile data will prevent the use of data across the board.

You won't get any benefit from turning on "assist" nor will you incur any downside or costs due to data usage.

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