As I can see that you've not declared to use this font anywhere on your site yet.
If you want to set this font globally, it's better to work with LESS file:
/templates/purity_iii/less/variables.less
Look for this section:
// Typography
// -------------------------
//@font-family-sans-serif: "Helvetica Neue", Helvetica, Arial, sans-serif;
@font-family-sans-serif: "Montserrat", sans-serif;
@font-family-serif: Georgia, "Times New Romance", serif;
@font-family-monospace: Consolas, Monaco, "Courier New", monospace;
@font-family-base: @font-family-sans-serif;
Then backup your site + compile less to css: https://www.t3-framework.org/documentation/bs3-configuration#compile-less