OK, I just had to make this post because I read countless posts trying to figure out how to get it configured with this router.
I am using a SnapAV Wirepath NVR and found the following settings to be successful:
UPNP: Off (both in the router and in the NVR settings)
Virtual Server: ON (port forwarding would only work when I was connecting within my network, as stated in another thread on this forum for another router - virtual server worked to connect externally)
DLINK DNS Relay: On (this is on by default, but thought I should at least state I left this on)
Port: 3xxxx (now, I don't think it's important to use such a wacky 5 digit number, but I do think it's important to NOT use port 80 which is the default for these)
Now initially, I could only connect within my network using all of the settings provided in the 'Home' screen of the Dlink router. However, and this is where maybe me being new to the whole networking thing steered me wrong, but you are NOT to use the Gateway provided on the Home screen. Your Gateway needs to be 192.168.x.x (for example, if your internal IP addresses are 192.168.0.xxx, then your gateway should be 192.168.0.1). Someone with more technical knowledge can probably correct me here, but the entire time it was not working was because I used the Gateway provided by Dlink on the main page.
That's all. Maybe this will help someone, maybe not. I tried to include the relevant key words here (SNAPAV the distributor, Wirepath NVR the device, Dlink DIR-890L the model number).