Changing icons in social menu

Hello, I customized the social menu and added various links. Some of them (Instagram, Twitter, LinkedIn) are shown as an icon, the other ones (OpenSea and the link to the site in german) are only shown as a link symbol.  I tried to use a plugin, but that does not help either. Although I have added icons to the links, they are still shown as I described before. Is there a chance to manage which icon is used with each individual link? The site is www.nf-artists.com Thanks in advance and kind regards!