How can I access routerlogin.net or IP after setting custom DNS (without reset)?

> I have a NightHawk RAX35 [...]

   RAX35[v1] or RAX35v2?  Look for "Model" on the product label.
Firmware version?  Connected to what?

> [...] and I just configured it to hit my pihole for DNS. [...]

   "configured it" _how_, exactly?

   LAN IP address of the RAX35?  IP address of the "my pihole"?

> [...] I normally would plug in to the ethernet port [...]

   "plug in" what?  A cable from some computer or other?  Which "the
ethernet port"?

> [...] turn off my router [...]

   What is your "my router"?  How is it connected to the RAX35?  (Hint:
If a device has different types of ports, then "connected to device" is
not enough detail.)

> [...] and hit routerlogin.net (for some reason the IP [address] never
> seemed to work).

   _Which_ "the IP [address] never seemed to work"?

   Now that we know what did NOT (seem to) happen, perhaps you might
reveal what, exactly, _did_ happen.

   "not work" is not a useful problem description.  It does not say what
you did.  It does not say what happened when you did it.  As usual,
showing actual actions (commands) with their actual results (error
messages, LED indicators, ...) can be more helpful than vague
descriptions or interpretations.

   In a properly configured network, it should not be necessary to
disconnect gizmo "A" in order to manage gizmo "B".

> [...] Now that url does not resolve correctly [...]

   It's the DNS name ("routerlogin.net") which is not resolved
correctly.  I suspect that that's because only your Netgear router would
know how to do that, and you've chosen not to use your Netgear router
for DNS.  But other explanations are possible, too, and my understanding
of what you connected to what, and how you configured your gizmos, and
from where you're trying to do what, is all very cloudy.

> [...] when I hit https://10.0.0.1 (the router IP [address]), it just
> times out. [...]

   How did you determine that that's the (LAN?) IP address of the RAX35?
Note that a router has two IP addresses: LAN and WAN/Internet, so "the
router IP [address]" is not a well-defined entity.

> 1. Why can I not access the admin directly using the IP [address]

   Wrong IP address?  Client device on wrong subnet or lacking a route
to the right subnet?

> 2. Is there a way to have this work while still keeping my piHole the
> main DNS

   Probably.  Whether it looks much like what you're doing now is in
some doubt.

> 3. (Bonus) How do I setup the Admin UI to be accessible from the WiFi?

   Wired or wireless should make no difference; the LAN is the LAN.

> If you're using the 10.0.0.1 address are you also running another
> router on the network? (whats the rax connected to for gateway)

   A fair question.  When you reveal what all your router-like devices
are, and how they're connected and configured, more progress might be
possible,

> It could be a double nat issue.

   Not NAT, but multiple subnets with inadequate/inappropriate routing
might be involved.

   Start again. Identify your (unspecified) "my router", and how this
stuff is connected.  Why do you want to use the RAX35 with your
(unspecified) "my router"?  Cascading multiple routers (configured as
full-function routers) can cause multiple problems.

   If your actual goal was better wireless-network coverage, then you
might want to configure the RAX35 as a wireless access point, and
continue to use your (unspecified) "my router" as your (only) router.

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

   If you don't have any client devices (other than the RAX35) connected
to your (unspecified) "my router", then you might want to disable its
router functions, and use it as a modem-only, with the RAX35 as your
(only) router.  (If you believe that you've done that, then you'd seem
to be wrong.)  Knowing nothing about your (unspecified) "my router"
makes offering specific advice about it difficult.

Sorry for the late response on vacation. As for the way the network works, the other "router" is the ATT fiber box (sorry don't have the model rt now). I tried to get a lighter weight brick but ATT said there wasn't one for 1gbs.

I did not setup DHCP on the piHole because I had externally acessible endpoints so I wanted to use the router. Can I change that after the fact without the router UI?

Thank you!