I had hoped that I could figure it out (get to really understand how and why) by myself, but after going through a lot of different sites and stuff I still could not do it. So, very, very thankfully I finally found this site with an “out of the box” working solution:

JR Nielsen website with the solution

I’ll get (some day) back to it and will try again to wrap my head around it.

This, by-the-way, works also with tax_query.