Add a comma-delimited list of RSS feeds (keep the http[s]:// bit in as well) to the list of feeds under Advanced > Edit Sources. (something like https://kewbi.sh/blog/index.xml,https://news.ycombinator.com/rss).
Matter only adds the latest 10 items to the feed - I personally didn't want to deal with reading or seeing more than that. If you fork your own instance, you can change this line to have a higher or lower value. (See self hosting)