Stop header being sticky on mobile for the Habitat Theme

[ConcreteLab]

Hi all.

I am trying to remove the "stickyness" of the header on my website when in mobile.

I have an upsell app and when the upsell popup appears on mobile, its behind the header and the "X" and "View Cart" buttons are hidden behind the sticky header on mobile.

I've reached out to the app dev but they are struggling to solve it, so the next best thing to my mind is to unstick the header.

The website is www.concretelab.co.uk. It occurs when you add this item to the cart - https://concretelab.co.uk/collections/gfrc/products/gfrc-concrete-worktop-premix-kit

Any help is very much appreciated.

Thanks!