Disabling menu on mobile

Hello, the standard wordpress menu wasn’t very good and was so tiny you could hardly click it. I decided to install the ‘ShiftNav’ plugin which i am happy with. The only problem is the old menu is still there so now i have 2 menu’s on mobile devices. I also have an annoying menu on the right side also that i don’t want and cannot figure out how to get rid of it.

The site in question is https://www.terraceshop.co.uk

I have googled this and tried all the recommended custom CSS but nothing works.