Hello - I think this would best be done with DAX.
Make sure there is an active, single direction relationship between the date column in your calendar table and the order date column in the Orders table. You will also need a couple of measures. Note, to use the measures below, your calendar table must also contain a column named Year Month Number. You can add this as a calculated column if needed.
Measure 1: # orders for the current month
# Orders:= distinctcount ( Orders[ORDER_ID] )
Measure 2: # orders for the prior month
# Orders PM :=
VAR CurrentYearMonthNumber = SELECTEDVALUE ( 'Calendar'[Year Month Number] )
VAR PreviousYearMonthNumber = CurrentYearMonthNumber - 1
VAR Result =
REMOVEFILTERS ( 'Calendar' ),
'Calendar'[Year Month Number] = PreviousYearMonthNumber
This was taken from DAX Patterns (Month-Related Calculations): https://www.daxpatterns.com/month-related-calculations/