Dir 655 problems

Hello,
when trying to install router to
at&t dsl modem cannot connect to
internet.following directions for
quick setup,it has me power dsl
modem down and move ethernet cable
from modem to router,power up router and wait.this part seems to work as i get a router config
complete prompt.the next step is
to connect ethernet cable between
router and modem and power up
modem .the dsl modem never shows
internet light.router also does
not display internet connection.
if i the same steps without powering down modem then router
and modem both display internet
connection,and my windows pc even
displays that i have internet
connection.but i get an error cant connect to internet display.
i can log in to router with no
problem