If you are working on ACC then you can upload the font inside Public Resources section and then use the MD5 path of that font on the CSS. Depending on the build you are on and the extension of this font, you might have to whitelist the extension on the server side (contact support if hosted by Adobe or contact your IT team if setup is on-prem).
If you are on ACS, please upload on a publically available site and Campaign will pick from there.