If you're able to connect via Sonic's modem using that same port on the Mac, the port should be good. Perhaps the port on the DM200 is bad. You can usually perform a hard reset (reset to factory default) by pressing + holding the reset button for 30 seconds while the modem is on.
1. Turn on modem, then wait for it to boot up etc
2. Perform hard reset (press + hold reset)
Wait a minute or two for the Dm200 to reboot and bootup
3. Ensure Mac is plugged in to the DM200
4. Try to login using the above methods mentioned earlier. (http://www.routerlogin.com or http://www.routerlogin.net or 192.168.5.1 in the web browser address bar)
1. Turn on modem, then wait for it to boot up etc
2. Perform hard reset (press + hold reset)
Wait a minute or two for the Dm200 to reboot and bootup
3. Ensure Mac is plugged in to the DM200
4. Try to login using the above methods mentioned earlier. (http://www.routerlogin.com or http://www.routerlogin.net or 192.168.5.1 in the web browser address bar)