ATT DSL and DVR anyone ?

ok Im pulling my hair out. Im trying to set up DVR to stream online and I have no luck. Im useing ATT DSL
do I use the gateway address of the router or the modem ?
should the submask be from the router or the modem ?
when it ask for IP on the DVR do I use the same as the router or modem?
I have tried each and failed. Do I need to go into the ATT siemen modem and change any settings first before hooking the router up? I can accsess the internet through the router. should I choose the ip of the router or modem. in the dvr setting going nuts may kill self.....