Are you sure you are using an old computer as a router?
Is it an ADSL connection? If yes, your ISP may require you to input username and password ***** authentication over PPPoE. I am afraid if your old computer can do that.
You may simply let a computer directly connect to the modem, make sure the computer is on DHCP, see if you can receive a public IP. If not, you probably need to be authenticated for PPPoE.