the problem is solved. I’m a little embarrassed, but it was my fault.
The issue wasn’t a firewall. It was a lot easier.
I have changed in the settings the version of Font Awesome from 5 to 4, but I’ve missed to renew the selection of the icons at the page. So at the page everything looks fine, but the website preview didn’t show the icons.
But I want to thank you very much for your help. I appreciate this.
Your plugin is great!