Error 721 is a microsoft windows error code which means thatyour computer cannot reach your ISP. There are several factors thatmay cause this error, please go through this list totroubleshoot:

LED on the front of your modem
Is the ADSL light ON/OFF/flashing ?
- It should be ON solid, in order to use the internet. If it'sflashing, the modem is currently 'training' meaning searching forthe ADSL service. If it's OFF, the ADSL line is not detected atall.

TCP/IP is not bound to your current networkconnection
Even though you computer communicates with the modem using USB, youcannot access the internet if the protocol TCP/IP is not installedand bound to that connection. Go toControl Panel -> NetworkConnections, right-click on Local AreaConnection and select Properties.Verify if Internet Protocol TCP/IP is installedand is ticked. If not, install it, enable it and pressOK.

Incorrect VPI/VCI settings
Your Internet Service Provider, or ISP, should inform you of thesevalues. Verify these numbers, and put them into the field, likeillustration shows.

Multi-link
Diasable this option for troubleshooting. This setting is bydefault set to disable.