Font Awesome icons change depending on the version. I remember when we did an update we had to redefine some that went missing. Maybe the ones you're looking for were dropped or moved to the "Pro" set. I see on https://fontawesome.com/v5.15/icons/hands-wash that hands-wash is pro, so wouldn't be available to us. If there's an icon that is for sure in the Font Awesome version that we're using but doesn't show up in the plugin, then apparently the icon set's definition file needs editing. But I just looked, and neither "cash-register" nor "hands-wash" (or any variation) is in the current themes/base_files/iconsets/default.php file.