Issue with modal over sticky section

I’d like to have a sticky nav on mobile. When I add a z-axis to the nav, I cannot set my signup modal to appear full-screen in front of the nav.

Page on load:
image

Sticky nav without z axis, lower sections cover it:
image

Sticky nav without z axis, signup modal fills screen:
image

With z-axis on sticky nav, modal appears behind:
image

I tried changing the z-axis of the flexbox inside the modal but no luck. I cannot change the z-axis of the modal itself. I tried just moving the modal down but it looks messy and doesn’t function as well.

Am I missing something? Not sure if there is a code fix, would need to be specific to mobile.

Thank you