Summary of multiple entites for each period in the report based on the prompt

We have a requirement that the summary of multiple entities for each period be reflected every month.

We understand that by inserting a formula column and adding a formula, let's say SUM([B]), where B has an entity prompt in the column, which will give us the summary. But the problem here is that we have a relative function used in the report, and when we apply the SUM formula, it sums up the period from January through the prompted period and entity, but we need summary data for every month for the entities that are chosen in the prompt.