We have a slightly (until now) undocumented feature with dynamic domains to allow you to create A records that point to your dynamic IP.
The value for the A record should be: ip. (That's two lowercase letters, "ip".)
For example if you wanted dynamic.example.com to be an A record pointing to your most current dynamic ip then you would create an A record for dynamic.example.com as type A with a value of ip and our system will always insert your most current IP into the DNS.
Another option would be to use a CNAME which is a perfectly fine solution. The solution presented above is often used by people who want a dynamic record called "mail.domain.com" which is not a CNAME so that it can be used for MX records. Similarly, NS records should list an address directly and should not be aliases.