Hello,
Navigate to Appearance > Menus > Create or select a menu.
Assign it to the main Menu (this will appear as your main site navigation).
If you want to use a Secondary Menu, add a secondary menu element in the header and assign the menu in the header.
For the mobile menu, navigate to Appearance > Menu > in the menu settings, set the display location mobile side menu.
Screenshot for clarification: https://postimg.cc/145FN2V0
To make the mobile menu full width, try to add the following custom css code in Theme Settings > Custom CSS > Custom CSS for mobile.
.mobile-nav.wd-side-hidden.wd-side-hidden-nav.wd-right.wd-opener-arrow.wd-opened {
width: 100%;
}
Best Regards,