Fontstack: Not working on small mobiles

Hello,
I am working on a new site with the Mountains theme. I use the Fontstack to pull in Google’s Open Sans font.
It is working fine on computers, but when I check with my small S7 (both with Firefox and Chrome), all texts are in the fallback font, not in Open Sans. Well, there are headers h3 for which I am using Font Pro and there I have a connection to Open Sans, which is shown on mobile.
When I use RW’s simulation view for iphone SE, everything is shown ok with the Open Sans font.

Any idea what can be done to resolve the issue? Thanks!
http://cp.rathmair.eu/

Below a screenshot of my Fontstack settings.