Everyone pretty well knows by now, I make a true mobile site mainly for the homepage, and when using Divi these setting on the phone styles seem to work best for me overall as people navigate to a responsive page within the site. Now when I make my mobile site, I still use some of the custom css setting in the rows of the actual mobile page, but these settings shown below are a great starting point. Access these setting in your theme customizer and get a jump start of the responsive look for your mobile site.