Dax Help!

I have a list of monthly recurring revenue payments and the start date/end date for these payments. I have a MonthList table and would like to show the SUM of all recurring payments that are within the range of the selected MonthName. 
There is already a relationship between the ManagedServices [StartDate] field and Calendar [Date] but can't figure out how to show all the "active" payments for the selected month.  

I know I haven't explained this very well but hopefully this makes sense!