Thumbnail won't show

So far I like the plugin. The install was easy and it works as I expected.
However there is one little issue I walked into.

I can create a new portfolio item. Which will show on the page I want it to. I can see and click it. All the captions are there as well.

But when I add a thumbnail (and a image) the link is broken. The link changes in the menu (edit portfolio item). It is changed to attachment_number. However the site doesn’t seem to recognise the link and I get a broken image icon and when I click it I get a “not found page”. I got one of the images to work (by changing the url option while uploading the image.) however this didn’t work for the other images or the thumbnails.

How can I make my images link correctly?
This is the link to my website:
https://colorfulmedia.nl/?page_id=131

Thank’s for any help you can give me.

https://www.ads-software.com/extend/plugins/awesome-filterable-portfolio/