I have a little difficulty in fixing the "Add to cart" block in the footer ONLY on mobile and ONLY after having scrolled its position. Anyone can advise me how to handle this? thanks

https://www.tsunaminutrition.it/creatina/490-creatine-micro-pure-zero-carb-500-g-3863349432812.html

At the moment I manage everything using a simple jquery but I have CLS problems and I am trying to figure out how to fix