A customer is new in a period if their first purchase ever falls in it, and returning if they had bought before.
Start with the first purchase date. REMOVEFILTERS('Date') looks across all time, while the customer filter stays:
First Purchase = CALCULATE(MIN(Sales[Order Date]), REMOVEFILTERS('Date'))
Then count the customers in the period whose first purchase is on or after the period's first date:
New Customers = COUNTROWS(FILTER(VALUES(Sales[CustomerKey]), [First Purchase] >= MIN('Date'[Date])))
VALUES gives the customers who bought in the period. Inside FILTER, [First Purchase] is evaluated for each customer (context transition), and MIN('Date'[Date]) is the period's first day.
Edge case: in January 2024, where the data starts, every customer looks new. That's missing history, not a sales miracle; say so on the report.