When you say you reset it to factory default, what process did you use?
I have an older Dlink router and had similar issues. I tried holding in the reset button during the boot process at power up. And pushing the reset multiple times to clear the setup. Eventually it worked.
The solution was "admin" for user and "password" for password.
Thanks for posting the solution. It has been a long while since I looked at that router, and remember something flakey about it. Like the of term BOOK WRONG. Book says admin and blank password. I know because I looked it up on their web site when I read your post. Then again it may be the Verizon firmware version of the router firmware, and they always default to admin and password. Glad you found your answer.