You are already logged in as the root user, there is no need to use su or sudo. The Broadcom program to get the DSL stats is xdslctl.
What do you want to do? Press the tab key twice and it should list all the commands. You can do quite a lot at the shell, but anything you change will not generally survive a reboot. For example, you could start another telnetd process, with a command like "telnetd -p 1023 -l login", then you would be enable to telnet to the shell over the LAN on port 1023, but that telnetd process won't be running after the device is rebooted.