Custom fonts and FontAttributes: how to apply Italic or Bold?

I work on a Xamarin.Forms project where I added 2 Google fonts: Montserrat and RobotoCondensed.

I can use these fonts without any problem in my app, with Labels or Buttons, on iOS and Android.

But when I try to apply a FontAttributes like Italic or Bold, this doesn't work.

I've first only added the "main" font ("Montserrat-Regular" and "RobotoCondensed-Regular"). Then I've added "secondary" fonts ("Italic", "Bold", "BoldItalic") but this doesn't change anything.

Is there something I'm doing wrong?