Most Welcome,

As your button shows little smaller in iPhone devices but shown as full width in android. To fix this used the same code again inside the below code starting and ending brackets:

Screenshot from iphone device:https://jmp.sh/dfbRLqb

@supports (-webkit-overflow-scrolling: touch) {
//paste your css code here.
}

In case after doing this if it still not works for you. Then share your admin login credentials so i can help you out.

Best Regards.