Make woocommerce product attributes searchable

Hi,

I am struggling to find the right setting to enable some dropdown checkboxes which will filter on woocommerce product attributes.
For instance I have created a search option on a result page
https://dabedoe.li-z.nl/alle-fietsen

the idea is that the filter “koppeling” is searching in the attribute koppeling for the value “zadelpen”
Actually one bike has this value , thus I should get 1 hit. However I am getting many hits.
What would be the way to define filters for woocommerce product attributes?

I guess I am struggling with finding the right search criteria in combination with setting the logic.
This product has a atrribute assigned
https://dabedoe.li-z.nl/product/weehoo-igo-turbo-aanhangfiets/

hope you can help?

regards,
marco