You should use media queries. Your fonts look too big on mobile devices. Once the element sizes scale down to fit current device screen sizes it will look better.
your send enquiry button stretches because side padding size stretches over the screen width of mobile devices. This is where responsive design comes in.
Also using absolute position for contact info is not a good idea in this case. Position them naturally in the flow without using absolute positioning. It will mess you up more in smaller screen.
For your menus,
why are you giving opacity:0? it’s invisible.
You want to create a hover effect using :hover selector to change its color and background color.