it is not stuck in mobile mode, the menu is the desktop menu and only switches to the mobile hamburger menu if you shrink the browser.
But it appears the site is set to a limited width of 700px.
There is a max-width:700px CSS on the body tag.
body {
background: #fff;
border: 1px solid #ccd0d4;
color: #444;
font-family: -apple-system, BlinkMacSystemFont, “Segoe UI”, Roboto, Oxygen-Sans, Ubuntu, Cantarell, “Helvetica Neue”, sans-serif;
margin: 2em auto;
padding: 1em 2em;
max-width: 700px;
-webkit-box-shadow: 0 1px 1px rgba(0, 0, 0, .04);
box-shadow: 0 1px 1px rgba(0, 0, 0, .04);
}
Check all the theme settings and custom CSS
If nothing in the theme, deactivate all non weaver plugins, and see if that changes anything. If so reactivate one by one