FA icons support

Shortly and briefly, my FA – icons for Menu don’t work. I get the option to put a name of the icon in Widgets -> Menu , but after reloading the webpage they just don’t show and I can’t add them in Customisation mode, even though i ticked the option to show fa-icon options in Menu. tried the local databse with icons, and the bootstrap as well. I really can’t stress out how unclear the instructions are.