I am very sorry but I am having trouble with finding why the website doesn’t show on tablet and mobile landscape, yet it shows great on desktop and mobile portrait…
Your Navigation Links Div has a z-index which is higher than everything else, causing it to appear above everything on these screen sizes. With a Position Fixed with 0% on each side, it takes up the entire screen.
You have a background colour of white set for this Navigation Links Div, and since it is on top of everything, it is hiding everything.