Assuming your lan ip is 192.168.1.x with gateway 192.168.1.1
You would start with your routers dhcp scope. I would suggest setting it to start at 192.168.1.11 to 254. This leaves you 10 ips to use statically.
Next you go to your dvr and set its ip as static 192.168.1.2 255.255.255.0 with gateway 192.168.1.1
You change the port from 80 to say 7000 [you don't want to use 80 for your dvr]
Save the settings.
Test the settings by going to your computer and in IE [or whatever you are using] type in the address bar
192.168.1.2:7000 [note the colon : ] and hit enter. You should have the dvr web interface come up.
Now you move on to external to your network access.
Go back into the router and port forward to 192.168.1.2 listing the port 7000. Save the router settings.
Go to the status page or wan interface and write down the wan ip your router presently has.
Now from a outside of your lan pc [friends house or internet cafe] put into IE that wan ip and port number just like you did when on the lan but replace the lan with wan ip
wanipaddress:7000 for example.
You should once again get the dvr interface.