Edimax router and serial cablemodem

I have an Edimax (BR6004+) router and a Demos serial cablemodem.
The router works fine when attached to an analog US Robotics 56k6 serial modem and is able to connect to an ISP.
Now I have a new Internet provider trough the TV cable with a serial (quite obsolete) DEMOS cable modem. The modem works fine (although quite slow) when connected directly to the computer. The connection is through a normal Dial-up adapter, dialing only number 4.
Attached to the COMM port of the router and after having configured the Internet connection settings (User, password, DNS's,..), the modem dials the provider but the router gives the error message "account name and/or password not accepted".
My questions are:
- Might the ISP block the connection after detecting I am doing it trough a router ?
- Do I miss any other important setting in the router ?
- Might the modem block any communication when detecting it comes from a router ?

Thanks a lot in advance for any help.

Jordi