Divi's Fixed Navigation Bar is only available on desktop screens by default. On mobile devices where the viewport width is less than 981px, the navigation bar will be static. 

If you would still like the navigation to be fixed on devices with small screens you can add the following CSS snippet to the Custom CSS option under Divi > Theme Options:

/* Fixed Navigation Bar on mobile devices */
@media screen and (max-width: 980px) {
    #main-header.et-fixed-header {
        position: fixed;
    }

    .logged-in #main-header.et-fixed-header {
        top: 0 !important;
transition: all .3s 0s;
    }
}

Here is the end result!

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