Hello, I've got a question :

I try to setup DKIM DNS TXT Records in the freeddns.org zone.
The DNS record with selector._domainkeys.DOMAIN.freeddns.org will not be published.
Is it possible to publish DNS Records with "subsubdomains" ?

Yes, you can have DKIM records for subdomains. You can add it in 'TXT record'. For more information, you can refer to https://www.dynu.com/Resources/Tutorials/DynamicDNS/CreateTXTRecord