Yeah, you just need to set your DNS using Cloudflare. Your router doesn’t matter. Ddclient will get your external IP address, usually by querying an external server like ifconfig.me (this is all configurable), and then use the configured provider (e.g. cloudflare) to point the DNS records to your external IP. You need to configure your DNS registrar to use the Cloudflare nameservers for your domain. Then just regularly (daily) run ddclient, e.g. using a cronjob