Holy crap! I followed the instructions in the modem/router's manual, then I quickly realized that my ISP (Bell Sympatico) has fooled around with my modem before (I'm renting the modem from them), and all the options of the modem are now locked. I can't do configurations on it, so I can't do the port forwarding trick.
I still wonder what made the difference: sometimes the port was blocked, some other times it was opened. If the port was blocked, what will the worst scenario be? Will I still be able to download, but only with a relatively slow download rate? I don't know what to do, sigh.
But the good news is that it is working fine now, for whatever reasons. I just worry that it will stop working properly again in the foreseeable future.