Apple has Wi-Fi assist on by default, which will switch you to cellular data if it deems the Wi-Fi network that you are connected to is too weak. Go to settings, cellular, Wi-Fi assist and turn it off. You can also monitor your data usage (iOS doesn't have a monthly counter so you'll have to reset it manually or use an app that'll monitor it on a monthly basis) to see what's using your data if it's still doing it without you knowing why. Let us know if you still have issues or needs help with anything else
Best answer by Zuya
hello, this morning when I've checked my data, it was around 250MB used. All day I didn't use any internet. and I came to home 6 pm, I talk to with my friend by video call and koodo sent me a msg that I used 954MB. I have wifi at home. Why my data is runing when using my wife? could please explain me, thank you!