XF 1.5 Configuring "Menu" items when viewed on a mobile device

I will be using this website as a guide: https://www.spigotmc.org/

When viewing the website from a phone, the navigation buttons on the navbar (i.e. home, forums, resources) become a single "Menu" button. When pressed, it will display a panel that reveals the rest of the navigation buttons.

I followed this guide by Jake Bunce to add custom navigation buttons. They can be seen on a computer, but when viewed from a phone, they don't show up on the panel that I mentioned earlier.

What else do I need to do to make sure the custom nav-buttons I've added can be seen on both computers and phones?

Thank you for any help you can provide.