Dear Guenni,
I have the same topic.
Upload just the Open_Sans 400 and Open_Sanns_600 ttf, woff, woff2 as a Zip – it seams that it does not work.
Also the CSS I am quite used and using since many years … here I find no way

@font-face {
font-family: myFirstFont;
src: url(sansation_bold.woff);
font-weight: bold;
}

Could you just help me out ?

It seams that its just the writing conventions boath
a) for the Zip-File …. ( each style separately ( did not work ) or together ( did not work )…)
b) for the @font-face { … } iNTEGRATION .. also did not work..

I would apreciate a very short help a lot because of a Go-Live-Deadlines with clients.

Many thanks in advance
Lisa