Orbi RBR50 set up as AP only help

> What order do I do this in? [...]

   Many things are possible, but I'd:

   Remove and reset the RBR50, and set it aside.

   Connect and configure the "my old Airport" for proper router
operation.  Without wireless, if that's your preference (but that could
be arranged later).

   Visit http://netgear.com/support , put in your model number, and look
for Documentation.  Get the User Manual (at least).  Read.  Look for
"Use the router as a WiFi access point".

   You should be able to connect a computer to the isolated RBR50, and
configure it as a WAP before connecting it to the main router.  It's
also possible that if you first connect the RBR50 to the main router, it
will look around, notice its situation, spot the opportunity, and ask if
you want it to be configured as a WAP.

   Either way, after you specify "AP Mode", and connect the RBR50-as-WAP
(according to the User Manual instructions), it should, by default ("Get
dynamically from existing router"), get its new/different LAN IP address
from the (DHCP server in the) main router.  Initially, you'd need to ask
the main router about that address.

   Once you see the details, you could arrange an IP address reservation
for it on the main router, so that it will always get the same
(memorable) LAN IP address.  I might need to read up on your version of
Airport to see how that'd be done, but I assume that it'd be easy
enough.

   Other variations are possible (like, say, "Enable fixed IP settings
on this device (not recommended)"), but using the default (DCHP) method
can make it easier to avoid errors.

   Normally, nothing much goes wrong.