Unfortunately, we can do nothing in case the problem independent of the theme.
Looks like it is a specific problem of the new version of ios. You will have the same situation with fonts even with a default theme.
Screenshots that you provided displays 2 different types of font – serif and sans-serif. I suppose Safari just does not display the font that should be – Lato and use some serif font.
We don’t see the described problem when checking your site with iOS 10. Unfortunately, we have not updated devices to iOS 11 yet to check the site with this version.
Can you, please, check and confirm that Lato font works fine with the default WP theme for you?