more data

How much data do you have in your plan?

As well, what kind of phone do you have? It should break down in your settings what app is using the data and potentially on which days. As well, if you have settings like wifi assist on, your phone will use your data over your internet if it deems it to be the better connection.

Turn off WiFi Assist. Turn off every app that you don't want to use cellular data. Turn off Background App Refresh for the apps that use cellular, but refresh is not necessary. Worked for me.

Also worth checking if you have an iPhone, iCloud backup over cellular can silently consume large amounts of data in the background. Go to Settings → [Your Name] → iCloud → iCloud Backup and make sure "Back Up Over Cellular" is turned off. Same goes for iCloud Photos sync.