Youtube cost me 300M from my data plan even though I'm hooked up to home wifi

[AC Question]

I just got a Samsung Galaxy 7. My plan gives me 1 G of data per month. I purposly chose the lowest plan because 95% of the time I'm either home or at work and can connect to wifi. But last night I watched some Youtube. It shows me that I've used 300M of data from Youtube. Why didin't it use my home wifi?

 

[B. Diddy]

Welcome to Android Central! Are you certain the wi-fi connection was stable/valid the whole time? Try turning off your mobile data altogether, connect to your home wi-fi network, and see if you get any dropouts or stuttering, which could mean your connection isn't stable. That might cause the phone to default to mobile data.