I'm using ZyXel VMG5313-B30A router and I've found in predefined list of tutorial setting Dynamic DNS GnuDIP option (dynu.com is not there) for update my IP address, but I'm did not success how to configure. On dynu.com tutorial pages I found that I can use proper GnuDIP for updating my IP address.
In main menu stay this for Dynamic DNS Setup:
Dynamic DNS Enable
Service Provider : GnuDIP
Hostname : *****.dynu.net
Username : ********
Password : •••••••
DNS server :???????

I don't know what is go in the last row DNS server?????

The DNS Server should be gnudip.dynu.com.

Yes, I've done already that, but nothing happens, router did not update ip address!!!!

flush your DNS