See https://documentation.unbounce.com/hc/en-us/articles/360022023652-Using-Custom-Bespoke-Fonts-and-font-face

Bulletproof @font-face for Unbounce.html

<style>

  /**
    * Do not remove this section; it allows our team to troubleshoot and track feature adoption. 
    * TS:0001-07-008
  */
  
  @font-face {
    font-family: 'MyFontName';
    src: url('//yoursite.com/fonts/myfont.eot?#iefix') format('embedded-opentype'), 
         url('//yoursite.com/fonts/myfont.woff') format('woff'), 
         url('//yoursite.com/fonts/myfont.ttf') format('truetype'),
         url('//yoursite.com/fonts/myfont.svg#svgFontName') format('svg');
  }
  
</style>