I tried that too on the DSL-300. Maybe yours works the same way.
1) Turn the modem's power off and connect it to a serial port.
2) Start up HyperTerminal (in Win2K, it's under Start, Programs, Accessories, Communications).
3) In Hyperterminal, define a new connection (just type any text and OK). OK the next popup too. On the next popup, set its parameters to the COM number where the modem is connected, speed at 9600, Flow Control Xon/Xoff. Keep Data bits=8, Parity=none, Stop bits=1. Click OK and you should get a blank window. Now Hyperterminal is waiting...
4) Turn the modem's power on.
5) The Hyperterminal window should display the modem's characteristics and stop at a line showing its IP number. That behaves like the DOS prompt. Enter HELP to see what's available. Some commands will take you down one level to their own menus. Type HELP again to see what these menus understand.
If you change something, use the SAVE command to save your changes (like the password maybe). EXIT takes you out (or shutting down Hyperterminal).
If you get garbage text on the Hyperterminal window, lower the serial port's speed to less than 9600.
Good luck.