Ok, I partially understand it now.
How I solved the problem:
Changed the Data Model in the Admin panel to change the column I wanted to filter from ID (Entity Key) to title (text).
Then, I changed the filter type from ID to Other categories.
and Voilá
I think the main problem here (a bug?/a feature request?) is that I cannot have a variable of type ID without making it a Filed Filter.
Anyway,
all looking good now.