You can’t delegate <name>.dedyn.io to an other dns service, this is not supported.
(If you want to manage your DNS zone at Cloudflare, you need your own domain name. But then I don’t understand what you want to use deSEC for. That’s like going to REWE but wanting to use Lidl. Both manage DNS zones. For your use case: Why do you want to use both at the same time?)