The way I use dial on demand is using pppconfig from the command line
do (ctrl+alt+f2) and get to a command line.
Type in pppconfig and enter your parameters for your ISP
go to advanced settings and turn on dial on demand. This will make your computer dial the internet whenever it needs to.
You will then need to type in
pon <the provider name you typed in>
and it will wait for you to try to get to the internet before it dials
to sign off type in
poff
and it will disconnect, unless you set a time out value in the advanced settings of pppconfig