What I find unusual is that you have your DVR connected to the modem and not the router. Therefore you have a direct connection to the Internet with your DVR and the IP address of the DVR would be issued by your ISP. Most ISPs allow the connection of multiple devices but limit the number of IP addresses they dish out. That is why most people have a router if they want to connect more than the ISP's limit.
Anyways, in this scenario your router has two IP addresses, the External IP which is assigned by your ISP and an Internal IP which is assigned by manufacture of the router and configurable by you. Your DVR would have another IP address assigned by your ISP.
IP address that start with 192.168.x.x are reserved for LANs and are not given out in the Internet world (there are others like 10.x.x.x). If you want to know what your WAN IP address is check out this site, it will let you know.
Now as for accessing your router from the WAN, you need to enable it. So from a computer that is connected to your router you need to change the settings and allow remote connection. You might need to establish a port.
Now as for you DVR, bring up the settings info screen and determine the IP address it has been assigned. Since you have it connected directly to the modem and you want to access it, you need to use that IP address. That IP should give you access to the DVR through the WAN.
Lookup the ip address of your computer and DVR, if they are in the same range then you'll be able to access it through your computer. 192.168.1.100 and 192.168.1.151 are in the same range. 192.168.0.100 and 192.168.1.100 are not in the same range (assuming your are using netmask 255.255.255.0)