iOS9 has introduced a new feature called "Wi-Fi Assist". This feature is designed to overcome weak Wi-Fi signals and allow your iPhone or iPad to automatically use your devices mobile data instead.
A problem with this is that you may not be aware of this and if you are updating apps or watching videos, you could very quickly chew through your monthly mobile data allowance.
The following steps will take you through disabling this option:
If you have multiple wireless access points around your home and wondering why they are not working as expected, click here to read more.