If you want to be able to upload custom fonts, you have to contact Fandom support to activate this.

Otherwise you can import webfonts in your wikis Common.css.

https://custom.fandom.com/wiki/Custom_fonts

Edit: ninja'd

If you can find the fonts you want in Google Fonts, then please see Help:How to import Google Fonts. If not, you should contact Fandom to enable WOFF2 files uploading in your wiki. You should also make sure you have a license to upload it.

Could you also provide a link to your wiki?

I tried it in the source editor, but it doesn't work.

@"Jay" AndrewBoy

The code needs to be in the page "MediaWiki:Common.css" to work.