Setting up second router as AP

> [...] when I do that, I cannot get to the web interface for the R7000
> to make the changes to it I need. What do I need to do?

   "cannot get to" it from _where_?  If your computer/device is
connected to your main router (C7100V), then I'd expect some
complications when trying to access the management web site of the
R7000, when it's still configured as a router.  The R7000-as-router
would see that as "Remote Access".

   Perhaps the easy way to do this would be to disconnect the R7000 from
everything execpt it power supply, and then connect one computer/device
(with a web browser) to it (wirelessly or LAN Ethernet port).  That way,
you should be able to access its management web site.  Do a settings
reset, if necessary, to ensure that it's in router mode.

   Now, "Select AP Mode", and "Apply".  When it restarts (in "AP Mode"),
connect it to the main router, as the instructions say.

   Consult an "Attached Devices" report on the main router, to see what
the new LAN IP address of the R7000-as-WAP is.  If all went well, then
you should be able to access the management web site of the R7000-as-WAP
at that address from anyplace on your LAN.  (A "routerlogin" name might
work from a device which is connected to the R7000-as-WAP, but its LAN
IP address should be good anyplace.)

   It might improve convenience if you reserve a (memorable) LAN IP
address on the main router for the R7000-as-WAP.  (Then restart.)  Look
for "Address Reservation" in the main-router User Manual.