Block Access Point Guest network access to WebUI or Only allow certain IP to access WebUI

I have a nighthawk r7300 that acts as an AP on my network. My main router has tomato shibby 1.28 latest build. The AP router cannot run tomato unfortunately. I am using a guest Network on it with wireless isolation turned on, yet it still can access my main routers WebUI. I have a password on it, but would still prefer if the guest Network could not access/see the WebUI at all. How can I accomplish this? My main router is 192.168.1.1 and the AP router is 192.168.1.2

Alternatively if this...

Block Access Point Guest network access to WebUI or Only allow certain IP to access WebUI