Fixed add to cart button

I have redirected directly the add_to_cart to the checkout page and have changed the text as per my requirement and now trying to fix the button at the bottom as sticky for a scroll of the content page only not the button. But it is not getting fixed the project is on localhost so not possible to give the direct link but the page link is given so to understand which page I am.

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