You are correct in your use of a "supporter report" You just have to use 2 columns in the supporter report.
Structure your supporter report in this way:
Col1 = Max(Month)
Col2 = Timestampadd(SQL_TSI_MONTH, -11, MAX(MONTH))
Filter: Month = user prompted month(s)
year = User prompted year
Then in your final report, your filter is based on both column #1 of your supporter report, AND column #2 of your supporter report like so:
Month <= Col1 of supporter report
Month > Col2 of supporter report.
Understand that this logic wont not work if the user selects wildly different months. ( 3 months from different decades )
have fun, please mark as helpful if this gets you on the right track. Otherwise post and I will try to clarify.