Theme "First" not adaptive anymore on smartphone when using domain forwarding

Hi there,

the domain https://www.vision-videoschool.eu is forwarding to the server https://vision.wettintv.de, where it is actually located.

The theme ‘First’ has an adaptive design, which normally changes by the size of the screen or the browser window.
When I open https://vision.wettintv.de directly from a smartphone the theme adapts to the screen size and changes the design of the menu.
But when I open it via https://www.vision-videoschool.eu it is not adaptive anymore on the smartphone (on a computer screen it stays adaptive when you change the size of the browser window)

Thx in advance!