FontAwesome 5 Free doesn't show in mobile devices

Hi.
I am having trouble implementing some fontawesome fonts in my template. It is very rare, but in the desktop version of the page the font is shown, however in the mobile version (not in Chrome Inspector, but on a real device), this font is not shown.

So that you can see an example, you can visit http://future.juanvillen.es, and see the icon that appears on the back-to-top. The same happens with shortcode-core plugin (with notices box)

Does anyone know what is happening?
Thanks so much.

It also does not work on Mac Chrome desktop

Do you have your font imported properly and available SVGs?
I didn't really bother to look for it, as included assets are a bit of a mess (CSS in head, JS in body)

Hi.

The fontawesome.min.css file is imported directly from main.css with @import url (fontawesome.min.css); and the fonts are in the webfonts folder (image is attached).
In fontawesome.min.css these fonts are imported:

@ font-face {
   font-family: "Font Awesome 5 Free";
   font-style: normal;
   font-weight: 400;
   font-display: block;
   src: url (../ webfonts / fa-regular-400.eot);
   src: url (../ webfonts / fa-regular-400.eot? #iefix) format ("embedded-opentype"),
     url (../ webfonts / fa-regular-400.woff2) format ("woff2"),
     url (../ webfonts / fa-regular-400.woff) format ("woff"),
     url (../ webfonts / fa-regular-400.ttf) format ("truetype"),
     url (../ webfonts / fa-regular-400.svg # fontawesome) format ("svg");
} 

Do I also need to include the FontAwesome JS files?

Not sure actually. It seems like some icons are just missing. Or all icons are not showing up?