Just now, Speedy has just sent a person named Ferry to help me fixing my Internet connection problem. The strange thing is that my Internet connection was fine about 1 hour before he came, so i can chat, browse, and download. Strange huh?
So he replaced my modem but a newer one (but a same model) and re-create my account and wait...wait... and voila... it can connect like usual. I tried to browse and try the new modem and so far it's working. Just when he asked me if i want to change the password, i changed my password (since i haven't changed it since first installation) and i reboot the modem, and when i restart it, it can connect again, BUT.... i couldn't browse anything. Strange right? It uses the same setting as the previous one, except that i change the password. Luckily, i spot the problem. He forgot to tick the NAT option, which is necessary in order to translate all of local request to 1 IP, which is my public IP or else i can't do anything (even pinging Telkom's DNS).
So... the result for now is that my problem is solved, but i will have to test it for a few days before i can give a conclusion that it has been totally fixed. But so far it's promising.