No video thumbnail with new theme

Hello, I’m using the “Times” theme since a few days. Unfortunately the video thumbnails don’t work in this theme.

The theme builder says: This plugin requires adding a custom code to display the thumbnail, it doesn’t overwrite the original thumbnail hence it isn’t shown on the page like a normal featured image would.

But isn’t that what this plugin should be doing?

This is the page where a video thumbnail is missing: https://www.marielouiseschipper.nl/category/lekkerbeeld/