I’m probably somehow missing something where to correctly put the CSS in my page.
I want to have no menu on my main page so since nothing worked I created an empty one and put it on my starter page with the “page menu” plugin.
The issue is in mobile view it still shows the menu ‘icon’ but there is of course nothing there to be shown.
So I read all across the forum and found the (many) CSS codes to hide the menu.

.page-id-35 a.mobile-menu {
display: none;
}
.page-id-35 .main-navigation {
display: none;
}
But regardless where I put it in the simple css on the page, in the additional CSS inside the customizer or (as my last resort) inside the style.css of the theme (in 18.0 theme options) nothing works.
Nothing won’t hide the menu because if I add one link to my empty front menu it is shown.