Either your IP is incorrect, or you have port forwarding issues.
Also, you really don't need to create multiple domains on duckdns. Create one, use that as the url here, and everything else will be a sub-subdomain. For example, you register mycustom as your duckdns subdomain, so you'll put "mycustom.duckdns.org" into url, and put "nextcloud,sonarr,sabnzbd" into subdomains and your services will be accessible at "nextcloud.mycustom.duckdns.org".
See here to troubleshoot the port/IP issue: https://blog.linuxserver.io/2019/07/10/troubleshooting-letsencrypt-image-port-mapping-and-forwarding/