Unfortunately there is nothing the SSG can do to change its behavior. The SSG and client only knows the address that the ISP gave the SSG so that is all it can report to DynDNS.
In this case your carrier is saving ip space by doing the NAT for many customers upstream of your physical device. So the SSG has no idea what the address is.
You will need to install a client software on a computer that reaches out to DynDNS to update the address so it goes through your ISP NAT to report the correct public address.
Also it is possible that the upstream NAT will prevent inbound connections to your SSG. This is an option on carrier grade NAT systems in configuration.