You may find it useful to make the mobile menu appear sooner than the default screen width. Add the CSS in the Divi theme options area. Change the max-width to whatever you like.
 
[css]
@media only screen and (max-width: 1320px){
#et_mobile_nav_menu {
display: block;
}
nav#top-menu-nav {
display: none;
}
}
[/css]