Isolate subnet with DD-WRT router

I have my ISP gateway (which is a regular wifi router) which is 192.168.178.1. This is connected to a DD-WRT router (using it's WAN port). I've had this setup for a while now and pretty much the default settings (Connection type Automatic/DHCP) worked. I set the local IP of the dd-wrt router to 192.168.0.1 and set DHCP to use the 192.168.0.xxx subnet aswell. This worked fine and I had an **isolated** subnet. That is: The 192.168.178.xxx subnet could not access machines on my 192.168.0.1 subnet, including the router, and vice versa. Now I got a new router, fresh dd-wrt installation, exactly the same settings and subnets, and suddenly I'm able to access 192.168.178.1 from my PC (which is on the dd-wrt subnet). Which setting is responsible for this, and how do I change that? Also, the new router displays the ISP router's domain name under WAN domain name (LAN domain name is empty). The old dd-wrt router did that too, but devices on the network where **not** on that domain (they were on an empty one instead). Now with this router, devices connected to it are automatically on that domain. I would really like to prevent that aswell. I hope my question is clear enough, and here are some more of my dd-wrt router's settings: Connection Type: Automatic Configuration - DHCP Hostname: Domain name: Shortcut Forwarding Engine: Enable STP: Disable Local IP Address: 192.168.0.1 Subnet Mask: 255.255.255.0 Gateway: 0.0.0.0 Local DNS: 0.0.0.0 DHCP Type: Server Start IP Address: 192.168.0.100 DNSMasq for DNS: Enabled DHCP-Authoritative: Enabled Forced DNS redirection: Disabled NTP Client: Enabled IPv6: Disabled entirely Advanced routing, Switch Config, Networking and Tunnels all on their defaults