Problem with the font avantgardebookbt on Mobile Devices (iOS)

Hello community,

I currently have the problem that in one of my projects on mobile devices Iphone and iPad the umlauts (ä,ü,ö) are displayed incorrectly they look like bold. On my Mac (in Mobile preview the problem does not occur). The font used is avantgardebookbt. Unfortunately I have no way to test the problem on Android.

Is this possibly because this font is not available on ios? What would you suggest as a solution? A fallback font or rather use a similar Googlefont?

Thanks for your help, and have a nice day everyone.

Here is my site Read-Only: *Webflow - MyGLO