So here's where I'm at in this adventure: Connected the DIR-868L to an Arris NVG468MQ WAN connection. Had WiFi from the 868L right away. Disabled the Arris radios. All good so far.
Then I went into the Arris DMZ screen. I enabled DMZ. A dropdown next to "Type:" defaults to "Share IP address with WAN." There is a grayed-out box next to "Default Server Internal Address:" Passthrough Mode: has 3 dropdown choices: DHCPS Dynamic, DHCPS-fixed and Manual. If I select "DHCPS-fixed" there is a dropdown with every single device in the house, including "dlinkrouter." So I haven't a clue about what to do in this screen. DMZ is currently disabled until I get more advice from you.
Also on the DMZ page is the following: "WARNING: Having DHCP Reservation enabled can cause problems with DMZ features. (DHCP Reservation is currently enabled.)" So I went to the Advanced - DHCP Reservation screen via a link on the DMZ screen. There is a button to disable auto-reservation. Under "Manual DHCP Reservation" there is a dropdown next to "Select MAC Address:" That dropdown has the MAC addresses for every device in the house. There is then a box that isn't grayed out next to "Manually Add MAC Address:" That box populates when I select from the dropdown with the MAC addresses. Finally, there is an item that says "Reserved IP Address:" Beside that is a list of ip addresses ranging from 192.168.254.10 - .210.
All of the computers and phones are connected to the 868L and able to access the web, so that's a good thing. So is there a problem with DMZ not being enabled?
Now to connect the USB network drive, which was the point of this exercise! [fingers crossed]