[Plugin: Yet Another Simple Gallery] need help with gallery main image display

Hi there. I installed the plugin (twice) and I have the following issue: the main image above the thumbnails don’t change when clicking on the them. Instead the page changes to the image link. The first thumbnail image is displayed as larger image permanently on the website page.

Any idea what goes wrong and how to fix? Thanks

https://www.ads-software.com/extend/plugins/yet-another-simple-gallery/