After setting R7000P as an AP unable to access admin page

> [...] I found that you must go to your 1st router and copy the Ip
> address that it has assigned to the R7000P, [...]

   That depends.  By default ("Get dynamically from existing router."),
the router-as-WAP will get its IP parameters using DHCP, so you'd need
to consult the DHCP server (typically in the main router) to learn the
new address of the router-as-WAP.  If, instead, you choose "Enable fixed
IP settings on this device (not recommended).", then you specify
explicitly the IP parameters of the router-as-WAP.

> [...] Also, I believe this might be the method that must be used by
> any Netgear routers that are set as APs.

   Netgear's and anyone else's.

   It's usually wise to reserve a particular (memorable) address in (the
DHCP server in) the main router for the router-as-WAP, so that it always
gets the same address, and you don't need to look it up every time you
need it.  On a typical Netgear router, that's be done at:

      ADVANCED > Setup > LAN Setup : Address Reservation
but something similar should exist on any router (DHCP server).

> [...] after going thought the forum, I found that [...]

   It's not immediately obvious that anyone with the problem will find
this thread, as opposed to the one where you found the solution.