R7000 as access point with Xfinity Security Router?

I've recently upgraded my Brother's Home Networking with a R7000 in access point mode.

He had two old Linksys Router running Tomato, which I configured years ago. Router #1 was 192.168.1.1 (Primary Router and DHCP server), Router #2 was 192.168.1.2 in "Client Bridge Mode".

So for the new router, I went to routerlogin.net and configured the R7000 in access point mode, I set the SSID's and rebooted the router. After the router rebooted, I was not able to login to the R7000 with the default passwd, and realized that I was hitting the logon page for an existing "Xfinity Security Router" (I think it's used to connect to an Xfinity Security Touch Screen Panel). I didn't want to make the R7000 primary, because I didn't want to break the connectivity to his Xfinity device.

Anways, the Access point's SSIDs are working from the R7000. Both regular and 5 ghz bands.

Question is: how do I find the IP address of the newly installed R7000? I cannot login to the Xfinity Router (Netgear WNR1000 v2), which is 172.16.12.1.

client devices are now getting DHCP IPs in the 172.16.100.x range. So, was I actually riding the Netgear wizzard from the R7000 or the WNR1000?

I spoke with several Xfinity Tech support reps and they could tell me the login info for the Netgear WNR1000 or tell me the exact function of this router.

Can anyone shed some light? I want to eventually make the R7000 the primary router, dhcp server, etc... Doesn't seem like an optimal setup right now.

There's also a Arris Surboard SB6141 that functions as the Cable Modem, which has the IP address of 192.168.100.1 which I can still access from a workstation.

Thanks for helping!