Hi Christoph,
So what device are you using to test this with? One way to test is to use Safari and under the developer menu you can change the user agent to impersonate an iPad - then visit the portal and see if you get the right desktop.
If you are using some other device then you might have to configure its user agent string in the portal DeviceGroup for Tablets so it sees it as a tablet when you access the portal.
Hope this helps,
Simon
