Hi there,
Please add the CSS below to your site via the customizer in Appearance > Customize > Additional CSS.
It should move the menu below the header on desktop screens. If it doesn’t, please post a url to your site.
Please note: the css uses Flexbox, which older browsers may not support, but it will work for ~97.7% of visitors.
@media screen and (min-width: 1281px) {
.site-header {
display: -ms-flexbox;
display: flex;
-ms-flex-direction: column;
flex-direction: column;
}
.site-branding {
display: -ms-flexbox;
display: flex;
max-width: 100% !important;
-ms-flex-pack: center;
justify-content: center;
}
.main-navigation {
float: none;
max-width: 100%;
text-align: center;
}
}