I am doing a site in Turkish which has some characters from the latin extended set. (https://sipsak.photography)
Since the built-in font Lato did not support the latin extended characters, I’ve changed my fonts to Open Sans. This fixed my font issues in the regular site. However, when I try using the mobile safari from iOS, those special Turkish characters are not displayed in the correct type face.
May be you need to load the subsets for that fonts. You can try installing this plugin https://wordpress.org/plugins/wp-google-fonts/, load the font that you want along with all the subsets and see if this helps.