Simple tips to make sure your Fontawesome 5 free icons work:
Make sure you’re referencing the correct font family and weight in your css:
.icon::before {
font-family: 'Font Awesome 5 Free';
font-weight:900;
}
Check the enqueued CSS version is valid for the icon you want to use:
function load_font_awesome() {
wp_enqueue_style( 'font-awesome-free', '//use.fontawesome.com/releases/v5.8.1/css/all.css' );
}
add_action( 'wp_enqueue_scripts', 'load_font_awesome' );