Announcement: Reminder - Digital Publishing Suite (DPS) Will End of Life on August 31st. More details are available here.
Make sure that your font license permits you to embed fonts in an HTML article. In the source folder of your HTMLResources.zip file, include a fonts folder that contains fonts and a CSS folder that contains the CSS styles file. In the .html file, refer to these files and fonts using relative paths. See Import HTMLResources folder.
You can also use a DPS App Builder setting to embed fonts in the app itself rather than in the article.