|What you write doesn't make a lot of sense.|
Modem converts cable signal to rj45.
Your statement " do not need any modem" has nothing to do with logging into the system and gaining internet access
I take it the "DIR-300" is a Dlink wireless router? Helps to include all the information.
"I then open 24 online client to connect to my ISP using username/password."
This sounds like a pppoe connection in that you have to authencate to the isps server before the isp lets you on the internet.
You need a router that supports PPPoE and its logon information on the wan interface. Your router has to do this authenication not your laptop.
Perhaps this link can shine some light on the issue for you