T-Mobile Data Usage When On Home WiFi

My bill is showing internet usage when I'm home and asleep in the early morning hours between 2am and 6am. The usage always ranges between 400mb to 700mb which is higher than the usage I use all day put together and I work from home e-commerce, use my phone for all of it too. But on my home wifi which is not with t-mobile. My back ground app refresh is off on apps. The phone updates automatically in settings. But that's not every night. I'd like to have a solid answer as to why this is. If they don't know, they need to get to the bottom of this. I also don't like the fact that there is no privacy with this company. I'm thinking about going back to Att.