Hi Aerion,
Thanks for your message, and welcome to deSEC! 
To use your custom domain with dynDNS, please send an email to support (including the domain name in question), and we’ll enable it.
Regarding the subdomains: The easiest solution will be to log into our web interface, and configure CNAME records at the subdomains that point to your main domain. This way, the IP addresses configured at your main domain will also apply to the subdomains (but you don’t have to update them all the time).
UPDATE (February 2021): We now support using the dynDNS update inferface on custom domains and on subdomains out of the box. You can use the full domain name (including subdomain) as the update username, and an API token as the update password. (Please note that if you want multiple subdomains always pointing at the same IP, it will be more efficient to create CNAME records for the subdomains using our web interface.) For details, see the docs. /update
Stay secure,
Peter