Thank you for your advice. However, only this code has solved the issue:

@font-face {
  font-family: FivoSans;
  font-style: normal;
  src:
    url("FivoSans-Medium.woff2") format("woff2"),
    url("FivoSans-Medium.woff") format("woff");
}
.fivosans-custom-class {
  font-family: 'FivoSans';
  font-weight: medium;
}

.fivosans-medium-custom-class {
  font-family: 'FivoSans';
  font-weight: medium;
}
.newsfeed .medium {
  font-family: FivoSans;
}

I had to add .medium, and now it works 😊👍 Thank you very much!