getting a Let´s Encrypt wildcard cert using dyndns and cname

[vince]

So, I found a lot of info on that matter, but I still don´t quite get it. I hope someone here is able to help me with that.

The setup is as follows:
1. we have example.com registered with a hosting provider
2. we have a subdomain sub.example.com with a cname record pointing to a subdomain with a free dyndns provider (freedns) at dyn.example.com
3. lets encrypt is set to DNS-01 using said dyndns provider
4. cert is set to CN sub.example.com and SAN *.sub.example.com

Testing this with the staging environment validation fails. The logs are empty. Any pointers/ideas?