You can check, that http port are setted correct. (Route on your DDNS or specify in get-query). By default port is 99, not 80!
Also, you can try to use /videostream.cgi?user=<USENAME>&pwd=<PASSWORD>
- it get you MJPEG stream instead FFMPEG, provided by videostream.asf
Finally, try to specify creditals in your request:
req.Credentials = new NetworkCredential("<USENAME>", "<PASSWORD>");
and try another query params: loginuse=<USENAME>&loginpass=<PASSWORD>