Can I assume that if I put a record "sub" as a CNAME pointing to ns4.dynu.com that this will work as a subdomain/host name and push traffic to "sub" to the dynamic IP address?

Thanks

Barry

You will need to list the following name servers for the subdomain name, sub.domain.com:

NS1.DYNU.COM
​NS2.DYNU.COM
​NS3.DYNU.COM
​NS4.DYNU.COM
​NS5.DYNU.COM
​NS6.DYNU.COM

If the current registrar does not have the infrastructrue for this set up. You can try signing up for a third level domain name *.dynu.net or any other available at https://www.dynu.com/en-US/ControlPanel/AddDDNSand add a CNAME record to point sub.domain.com to the *.dynu.net domain name you signed up.