Thank you. I have followed your steps, and can report the following:
1.Theme Related Issue
When I switched to the Twenty Twenty-One theme, the menu reappeared on mobile. Meaning the issue is theme related.
According to the instructions: If the problem goes away, it is something related to the X or Pro theme, and you should continue the troubleshooting.
2.Plugin Conflict
Deactivated all plugins, including Ubermenu:
Of course, No menu appears at all.
Deactivated all plugins, except Ubermenu:
Problem persists on mobile (menu is still blank)
3.Theme Update related issue
Theme is already updated.
4.Child Theme Related issue
When switching to the parent theme, the problem disappeared. I don’t see anything in the customizations that have anything to do with Ubermenu. Could you take a quick peek? (I’ll post that code in the secure note, for your convenience).
5.CSS/JS Customization
CSS – didn’t change the behavior
JS – there aren’t any customziations
6.Disabling Cache
N/A
7.Disabling CDN
N/A