I agree Cuddles that Kppp is usually excellent, but when it fails to work for whatever reason you need to be able to find another way if at all possible.
Loi, what is your output from

tail -f /var/log/ppp_article ? (assuming you followed the articles)

At a guess does your /etc/ppp/peers/internet file contain the usepeerdns option?
e.g. does it look something like this

/dev/modem
115200
modem
lock
connect /etc/ppp/chat-connect
user loi
defaultroute
usepeerdns

regards,
Mark