How to set the Data Billing Cycle to one day in Nougat?
I reckon it is difficult to adjust the billing cycle to 1 day (if possible). However you can try third-party applications such as 3G watchdog, which is much flexible in terms of adjusting billing cycles (daily/weekly/monthly or custom cycles)

For instance, you can set a daily data plan which of course expires on next day, to start a new cycle (with option to carry over any unused data to next period:



Additionally, there are widgets which reflect current data used, and amount left.