Adding an icon to the header

Hello,

I have been trying to add custom icons to text areas using <i class="icon icon-lhf_iconlibrary_online-lg_lungcancer-grey"></i>

While it shows the icon on the editor, it does not display these custom icons on the live version. I tried it with a Font Awesome icon and it does display that one using <i class="fas fa-adjust"></i>

I don't know why and I am hoping you can help. This is the page where I am currently trying to show it. When inspecting I can see it is there, but it's not visible. https://lhfstage.wpengine.com/lung-disease/a-to-z/lung-cancer/