Modal not working on mobile

The add-on works fine from desktop, but the modal doesn’t show on mobile.
When I click on the “free download” button, nothing happens, or after some tweaking (I don’t know what I did), the page refreshes.
I tested it on two smartphones.
I had to disable the add-on and now the free download link leads to the checkout page.

The page I need help with: [log in to see the link]