Hey @FPJ
Welcome to the Community 
To be able to use Wi-Fi connection, you need to have the option turned ON in your phone settings and to be connected to a working/valid Wi-Fi.
If one of those element is missing, your device will use your mobile data for any data request.
This means that you probably used your cellular data instead of your Wi-Fi connection since it was turned off.
Don't hesitate to keep an eye on that option before any data session.
