Add icons to Divi mobile menu items
To add icons to Divi mobile menu items go to Divi -> Theme Customizer -> Mobile Menu Customizer -> Menu Icons Settings and find the ENABLE/DISABLE MENU ICONS setting.
Select Enabled to activate icons for the menu and start adding icons to each menu item using the SELECT MOBILE MENU ITEM setting which displays the list of the menu items.
Selecting a menu item reveals the SELECT ICON FOR: setting with the list of icons (native ETModules icons set of Divi) to choose from.
Icons can be added the same way to the secondary menu items as well.