The Fritz!Box online UI provides information about the data that has been downloaded "Today" / "Yesterday", "Current week", "Current month" and "Previous month". Please find the statistics in the menu at "Internet -> Online Monitor -> Online Meter".
Unfortunately, it seems like this information is not exposed via the API. The only metric that gets close is "NewX_AVM_DE_TotalBytesReceived64", but this will be reset after a restart and is useless in this regard.
Obviously, this time-based statistic has to be saved somewhere. Is there any chance to retrieve this via the API?
Thank you so much!