Thanks for contacting us.
Simply go to the Dashboard >> Appearance >> Menu then select the mobile menu and then create/edit the menu as per your need like this I added a sub-menu item in the mobile menu on our test site see image: https://postimg.cc/jwy2FbCZ and now check the resulting image it appears fine: https://postimg.cc/Yj9qFYzd
Regards 8Theme Team.
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