Your results page was filtering based on a Category column, not Category_ID. That is why I was directing you to use that column instead.
If your products table does have a Category_ID column, then you would have to update the results page to allow filtering based on that column as well by editing the Database Search server behavior.
You also have to include &Search=1 at the end of the URL to make sure it initiates a search. Please post a current copy of the pages with the updates I recommended so I can see what might still be wrong.