Double nat error on xbox but only have one router?

> I have a new internet provider [...]

   Is its identity a secret?

> [...] I only have a small power brick that comes into the house, [...]

   Not a very detailed description of anything.  Some gizmo with an
Ethernet port?

> [...] I was hoping there were some setting I could change in the
> router itself to help me out?

   Probably nothing you can change, but you could check the IP address
of the WAN/Internet interface of your router.  I'd try:
      ADVANCED > ADVANCED Home : Internet Port : Internet IP Address

although the RAX80 User Manual says:

      You can view the router's Internet IP address on the BASIC Home
      page.

   Visit http://netgear.com/support , put in your model number, and look
for Documentation.  Get the User Manual (at least).  Read.

   Plug that address into the form at: https://whois.arin.net/ , and see
if it is a public or private address.

   The usual cause for that to be a private address would be connecting
your router to some other router -- either a device on your premises,
or one at your (unspecified) ISP, if they use carrier-grade NAT.

      https://en.wikipedia.org/wiki/Carrier-grade_NAT

   If your ISP is causing the problem, then you could try asking about
getting a more direct connection to the Internet.