Our theme is responsive design. You can view demo as there is already a different menu with three line on responsive view.
Once goto dashboard -> appearance -> customize -> layout options -> responsive layout and check your responsive is off or not. If it is OFF then make it ON
Yes I have responsive layout on. It all works on mobile devices but my issue is when it resizes my banner it makes the text I have as part of my image look to small. If I could upload my own mobile banner which I can design to have less text in the image it would look better. I hope ths make sense?
Seems like your header image font is a part of the image and the font will also resize with the header image while viewing it from mobile devices. You can not set individual mobile banner. It needs code customization you have to hire a developer as it is a new feature to add in the theme.