Magento – Magento2 – Layered Navigation on category with children

I am fairly new with Magento 2 and never worked with Magento 1. I'm working an a Magento 2 theme now, which includes a sidebar with price slider and category filter. Magento 2 supports this (almost) with Layered Navigation.

My question is, why does the Layered Navigation not show up on category overviews with children. For example I want to add it to this page: http://sebwite.hypernode.io/men.html.

I'v created my own block with price slider (so without Layered Navigation). This is a simple form with GET request. But when the param price=20-50 is added to the url, nothing happens in my theme. If I activate the Luma theme, the products are sorted.

How can I create a price slider that will work on every category page? Do I need to add an attribute filter myself to the product collection, or is there an easier way?

Thanks.