"You need to put the ST516 modem in bridge mode, and configure your Belkin router to handle the PPPoE login by entering your username & password in it. That will allow the port forwarding settings in the Belkin router to work properly."
Yes, you are correct that my ST516 modem is handling the the internet login(PPPoE).
But how do you put the ST516 modem in bridge mode? And I always thought you're able to login into your modem and router with the PPPoE login..