You may or may not know, the latest version of iOS has something called ‘WiFi Assist’.
Essentially if your WiFi signal becomes weak, it will switch your phone automatically to use your 3/4G connection without any kind of notification (yes, it is a bit snidey!), so you could find yourselves running out of your data allowance while thinking you’re connected to WiFi.
To check if you have this, go to Settings > Mobile Data and scroll right down to the bottom of the screen, where you should see WiFi Assist. You can then turn it off to help prevent you going over your data allowance.