RSS feed doesn't show any featured images

Hello,

I would like to add the posts feauted images to my RSS feed: https://fit4livin.dk/feed/. I’ve tried several plugins, see eg:

Featured Image In Rss Feed: https://screencast.com/t/y4CkgXzF
Featured Images In RSS Feeds: https://screencast.com/t/D4ZQwZwDbS
RSS Image Feed: https://screencast.com/t/eGf5rKr1

But none of them works. Is there something I’m doing wrong? Hope someone can help ??