You cannot use lod to get a fixed aggregation of a value computed using a table calculation, which is the case since 'Within 30 days' use the lookup table calc.
But if you look in the attached, you can still get what you are looking for using only table calculations.
First , I modified your 'Within 30 days' to return the Customer Name , and I nested this calculation in a Window_Count (computing on MY,Customer restarting every Month).
Next , I removed your filter in order to keep all customers on the row shelf, because I need them to have one mark per customer per month. This allows to compute a filter returning only the first row, showing the window_Count.
Finally, I unchecked 'Show Header' on the customer pill.
Thanks a lot for your solution! This seems to work