Hey there,
Looking at the code it disables for screens smaller than 600px, the iPhone being one of those.
You’ll not that it’s enabled for:
@media screen and (min-width: 600px)
So we could reverse that like:
@media screen and (max-width: 600px) {
.main-navigation ul.nav-menu,
.main-navigation div.nav-menu > ul {
border-bottom: 1px solid #ededed;
border-top: 1px solid #ededed;
display: inline-block !important;
text-align: left;
width: 100%;
}
.main-navigation ul {
margin: 0;
text-indent: 0;
}
.main-navigation li a,
.main-navigation li {
display: inline-block;
text-decoration: none;
}
.main-navigation li a {
border-bottom: 0;
color: #6a6a6a;
line-height: 3.692307692;
text-transform: uppercase;
white-space: nowrap;
}
.main-navigation li a:hover,
.main-navigation li a:focus {
color: #000;
}
.main-navigation li {
margin: 0 40px 0 0;
margin: 0 2.857142857rem 0 0;
position: relative;
}
.main-navigation li ul {
margin: 0;
padding: 0;
position: absolute;
top: 100%;
z-index: 1;
height: 1px;
width: 1px;
overflow: hidden;
clip: rect(1px, 1px, 1px, 1px);
}
.main-navigation li ul ul {
top: 0;
left: 100%;
}
.main-navigation ul li:hover > ul,
.main-navigation ul li:focus > ul,
.main-navigation .focus > ul {
border-left: 0;
clip: inherit;
overflow: inherit;
height: inherit;
width: inherit;
}
.main-navigation li ul li a {
background: #efefef;
border-bottom: 1px solid #ededed;
display: block;
font-size: 11px;
font-size: 0.785714286rem;
line-height: 2.181818182;
padding: 8px 10px;
padding: 0.571428571rem 0.714285714rem;
width: 180px;
width: 12.85714286rem;
white-space: normal;
}
.main-navigation li ul li a:hover,
.main-navigation li ul li a:focus {
background: #e3e3e3;
color: #444;
}
.main-navigation .current-menu-item > a,
.main-navigation .current-menu-ancestor > a,
.main-navigation .current_page_item > a,
.main-navigation .current_page_ancestor > a {
color: #636363;
font-weight: bold;
}
}
You’ll need to test this to see how responsive and well it will work on devices smaller than 600px.
Another option would be using Jetpack, that has a mobile theme included in there which is pretty sweet:
https://www.ads-software.com/plugins/jetpack/
Hope this helps.
Take care.