register a previously purchased router ac1750

> register a previously purchased router ac1750

   "AC1750" is a speed, not a model number.  Look at the product label.

> Model: R6400|AC1750 Smart WiFi Router

   Is that accurate?  Not an R6400v2?  Look at the product label.

> [...] They ended up not using it and gave it back to me. [...]

   Did they (or anyone) play with its settings?  If so, then a settings
reset would probably be a good way to start.

   Visit http://netgear.com/support , put in your (actual) model number,
and look for Documentation.  Get the User Manual.  Look for the LED
descriptions and "Troubleshoot".  Look for "reset".  Further reading
might be helpful, too.

> [...] I did the ipconfig commands on my computer [...]

   That could mean almost anything.  As usual, showing actual actions
(commands) with their actual results (error messages, LED indicators,
...) can be more helpful than vague descriptions or interpretations.

> [...] and I get an IP address. [...]

   Which one?

> [...] Thought that I needed to have it registered to clear up this
> issue. But you are saying that's not necessarily true.

   Read the User Manual.  Does it mention registration?

> [...] none of my wifi devices (phone, tablets, laptop, etc.) are able
> to connect via the router. [...]

   What about wired devices?  "not able" is not a useful problem
description.  It does not say what you did.  It does not say what
happened when you did it.  See "As usual, [...]", above.