We do not provide support for hacks, so use them at your own risk. If you're not comfortable with coding, we recommend against using this hack.
For best user experience, Semplice disables the sticky effect on the navigation bar for mobile. However, with a line of CSS you can make it sticky again.
In Semplice, navigate to Customize > Advanced > Custom CSS and place the following line of CSS:
.is-frontend .semplice-navbar {position: fixed !important;}
Once placed, you will now have a sticky navigation.