[Oni]

Yes i am, with the mobile version, i can't even see the right side of the page, the Legacy theme however doesn't have that problem

For the Modern theme you'll want to disable desktop mode, as it is responsive. Legacy is not responsive.

if i disable the desktop mode for the modern theme, i can't see a big part of the page, especially on the index page were there i can't see half of the tables

What's your phone model/screen resolution? Browser?