Here's a brief description of what ddns is about - http://www.internet-computer-security.com/VPN-Guide/Dynamic-IP-Addresses-VPN.htmlI don't know anything about networking but what's the benefit of having that dns on or off ?
TI's xxx.mydns.telkomsa.net option provides ddns automatically when using your TI account. Problems arise when using multiple ISP accounts as TI only knows the last IP they assigned you thus the need for the services of 3rd party providers; DynDNS, No-IP, DtDNS and the like. Note that most modems only cater for specific DDNS providers - usually "paid for".