Nicolas,
Am I supposed to make that or is it already made?
There is one already made inside Display > Content Menus, called "Products listing" and it is set to show products from "product category" (and my 2 custom categories are inside that), so I gave that ID to filter module but it doesn't work normally.
Whatever I choose in the filter, it just returns the 2 CATEGORIES, pics and fruits (and no, I did not put in the filter module the ID of the category listing by accident, I made sure it is the ID of the product listing that was in content menus).
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OK I made it work.
The product listing menu was not assigned to any module, for some reason I clicked that and assigned menu listing to that with the same id, which then turned into a module, for which I chose no position but to appear on every page, and now it works (and the "module" is not there in the modules section of joomla).
I don't know what happened exactly but it works, but I think this might be overcomplicating it.