The easiest way to get the IP of your DVR device is getting it from the router's administrative interface. If the DVR is set to DHCP, then the router will provide the same IP address unless it's requested from another device. Otherwise, it switches to something else. To get its IP, go to DHCP Clients in the router's menu and you should see the DVR's IP address. Alternatively, connect to the Administration interface of your DVR and check the Network settings. Most of these devices have DHCP enabled to obtain the IP.