Hi Chrisoph,
Webflow’s custom attributes feature is for setting attributes on an element, it doesn’t have any relevance to the collection list filtering feature.
What is it that you’re trying to accomplish?
It sounds like you have a CMS Collection and the ECom Products collection, and you’re trying to display a set of products on your CMS collection page that relate to that item?
If so a ref field is probably the way to go. You can put a custom field in your ECom Product collection that references your CMS item, and it will be easier to admin and filter by.