I think all you should have to do is make sure the "Dial whenever a network connection is not present" box is checked. When the connect dialog pops up, make sure you have it set to save your password and/or automatically re-dial and all that bullshit. It should automatically re-dial whenever it needs to then.

Dunce was a great program back in the day, but it's kinda outlived it's usefulness now. Windows DUN now pretty much does everything Dunce used to do. As a result, Dunce hasn't been developed in over 5 years, and their website says it only works on Wi***, and IE versions pre-4.0

http://www.vecdev.com/dunce.html