Call a Query on selecting an item from a Dynamic Select List

refer this image:

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the select list on left with 2 items (CB1, CB2) is dynamically generated.
the grid on right has dynamic repeat boxes with 2 items shown in each box from the database.

what we want to do is, on selection of CB1 from the list, the GRID on right side should filter with items which fall under CB1.

how do we set a dynamic action to run a query on selection of the one item from the list which will effect the view on the right side grid.

please help.