how can I set Roboto Slab to italic using fontspec

I am having the weird behaviour, that I cannot get a given font to be italic. Bold is no problem, but italic is.

RequirePackage{fontspec}
defaultfontfeatures{Mapping=tex-text,Scale=MatchLowercase}
setmainfont{Roboto Slab}[
        Ligatures={TeX},
        %ItalicFont=Roboto Slab Light, % I can set another font, but seem not to be able to find THIS font in italic.
]

when I now do

textsl{text} ... or ... itshape mytext

nothing happens.

How do I get the font to italic.

documentclass{article}

usepackage{fontspec}
defaultfontfeatures{Mapping=tex-text,Scale=MatchLowercase}
setmainfont{Roboto Slab}[
  AutoFakeSlant=0.2,      
  Ligatures={TeX},
]

begin{document}
  textsl{text} ... or ... itshape mytext
end{document}