View detailed data usage/consumpotion on an iPad (running iOS 11 or later)
I'm not sure about iOS 11, but iOS 12 uses MB as the unit if the data consumption is under a GB.

With the cellular data counter in the built-in Settings app, you are limited to the statistics shown and the option to reset counter. There's no way to further breakdown on a day/week/month basis.

You can try searching on the App Store for 3rd-party data monitoring apps. Some service provider provide their own app which tracks data usage over a period of time. If available, it would be best to use it.