Hello Andrew,
Thanks for giving us complete information.
To achieve a horizontal submenu please add the following code in your Customizer via Appearance > Customize > Custom > CSS:
.x-navbar .desktop .sub-menu{
display: block;
min-width: 270px;
width: auto;
}
.x-navbar .desktop .sub-menu li.menu-item{
display: inline-block;
float: left;
}
Hope this helps.
Thanks.