Whatismyip.com will give you your external IP that is reachable from the outside( anywhere other than your local network.) That is indeed what you're looking for.
and yes, that includes anything on the same modem.
Heres what you do from start to finish:
Find the address of your DVR. Its likely 192.168.1.X
In your router settings, set up port forwarding on the appropriate port to that address.
Access your DVR by typing in the address you get from http://www.whatismyip.com followed by : then the port.