Today at the HelpDesk we were dealing with a corporate customer who was experiencing HIGH mobile data charges and wasn't able to pin down the cause. We had a pretty good idea of the cause and this was confirmed when we took a look at one of the mobile handsets with high usage. In IOS 10 Apple introduced a new 'feature' called Wi-Fi Assist which is supposted to increase mobile data reliability for customers with poor wifi, which is great, but the issue is that even if you make sure you only use traffic intensive App's like YouTube etc when your on wifi, with WiFi Assist enabled the device can and will use mobile data (without telling you) if your wifi signal becomes weak, and that's ok if you have an unlimited data plan but we all know those don't exist in any form.
Turning it off is easy if you can find it, go into setting, then mobile data (towards the top) then scroll all the way down to the bottom and there is it. in the example below, Wi-Fi Assist had assisted us to use 478K of mobile data whilst we were on Wifi. Whilst your in the screen and have turned off Wi-Fi Assist then its worth having a look through the apps listed to make sure you've allowed/denied mobile data as needed.