On days where I have a large data usage (>100 mb), I typically have a large block of usage in the middle of the night (2am - 5 am). This is a time where I am asleep and connected to my own wifi. Where is this usage coming from?
You probably have Wi-Fi assist turned on. It's on by default. It will switch you to cellular if the device considers the Wi-Fi "weak." Go to settings, cellular, scroll until you see Wi-Fi assist and turn it off.