Menu doesn't work on Mobile

Good day. I have the problem that the menu on my website does not work when I go to my website on my cell phone. I’m actually very happy with Alizee, but it’s going to be like this for a year now. I thought at some point there would be an update that would fix the problem, but none came. That’s why I try support. Here is my website: www.sv-maegenwil.ch
PS: Sorry for my bad English. Used Google translator

Hello there,

Thank you for contacting us.

Try adding this CSS code to Appearance > Customize > Additional CSS from dashboard.

    @media only screen and (max-width: 991px) {
      #site-navigation .menu.mainnav-mobi {
        display: none;
      }
    }

Regards,
Kharis
aThemes Support