If unable to access the device using the DH DDNS host name, first verify that the ports are forwarded for the device (external IP address) using a website such as http://www.yougetsignal.com/tools/open-ports/
If the HTTP and TCP ports are not open, recheck the router(s) at the local network to verify the port forwarding rules are still valid. For more information on port forwarding, please visit the Network page.
If the external IP address is accessible remotely, but not the DH DDNS host url, check out the DH DDNS tutorial here. Double check the information input into the device. Check to make sure that all other DNS options are disabled except DYNDNS and make sure to enter in members.dhddns.org instead of members.dyndns.org.
If all information is correct and the DH DDNS host still does not function properly, this is the result of newer firmware not allowing for DH DDNS to function properly. The most up to date firmware for network IP cameras also does not allow for DH DDNS to function. Dahua suggests using QuickDDNS instead. Check out the How to use QuickDDNS article for more information.