1721 and Speedtouch 780

[Henry]

Because of the problems i'm having with the stability of my ADSL2+ connection I want to use a Speedtouch 780 as a modem. I have set up a DSL connection between the 1721 and the 780, and the 780 is connected to the internet, but I can't seem to connect my pc to the internet.
How do I have to set up both the 1721 and the 780 to get this going?

Regards,

Henry