You can try to add a class to <nav role=navigation and the try to style it like at W3C
https://www.w3schools.com/howto/howto_js_navbar_sticky.asp
or use a js like https://github.com/MonikaManduk/Responsive-nav-fixed-top (there are also others) – i have not tested the evtl. upcoming or remaining problems (evtl. overflow hidden for the content etc.).
