Which search indexers are you using in radarr/sonarr?
DHT allows discovery of torrents by pinging the IP addresses from an existing torrent, and asking them what other files they’re sharing. It then pings the other IP addresses seeding those files, and asks them what they’re sharing, and so on.
You can either use a torrent search index site (many of them use DHT to create their database) or you can self host your own DHT crawler and have your own personal torrent search index, but the downside is it uses a decent amount of space to store the index.