This will not work when an unmasqueraded 192.168.1 device such as the Pi-Hole accesses the Internet. The ISP router will masquerade the 192.168.1 IP to the house's public IP. The problem occurs after the response from the external server has been de-masqueraded back to the Pi-Hole's 192.168.1 IP. At this point the ISP router does not know what to do with the packet, since it does not have a route to 192.168.1.0. There needs to be a way to add routes to the ISP router's routing table in order for a non-NAT secondary router to be feasible.