How do I get my custom font to display on a smaller screen?

Is there an easy way to make sure that my custom font is displayed even when the screen size is smaller? Or if not, is there a way to make the fallback font Helvetica/sans-serif?

The custom font currently works, but only if the screen is above a certain size. If the screen is too small, then the default looks like Times New Roman, which I don't want. The code I'm currently using: 

@font-face {
 font-family: 'sanz';
 src: local ("sanzmedium");
 src: url('$assets-sanzmedium-woff');
font-weight: normal;
}