Google fonts not displaying on Safari 12/macOS Mojave in combination with Wordpress

Good Evening,
I have a problem with google fonts in combination with safari 12 on macOS Mojave and all my Wordpress-installations.
Somehow, safari only uses the fallback, Helvetica or Times New Roman. I have the problem with every instance where I used google fonts on a self programmed Wordpress theme (based on _underscore) where I simply included the font face directly via the google fonts api in the head-section.
I already tried to include the font-faces manually with woff, eof, eot and ttf as alternatives, but nothing seems to work.

Even more strange is, that it works on other sites I made without Wordpress, but I can't figure out what is causing the problem. It works on safari on iOS, chrome and firefox without any problems.
Example: http://2018.fcforst.de/ --> Should use roboto and roboto slab
http://konsolen-news.net --> Montserrat in the menu
Does anybody else have similar problems or a solution to this?

Thanks for your time.