Filtering within a subcategory?

I would like site visitors to be able to filter within this category of posts.

The category is resources (which is easy to display)

However I can’t work out how to allow filter by subcategory e.g. youth, community engagement

I think it should be possible with this plugin but can’t seem to figure out the shortcode. Anyone know how to do it?

So when you load the page all the posts from Resources will show up, and there is a dropdown where the user can filter by Resources > subcategory. I don’t want the other primary categories to show up though, e.g. News etc.

I know I could do it with a custom post type but they already have two of these and it’s not a big site!

The page I need help with: [log in to see the link]