How do you have it connected to your existing router? You should be connecting one of the "inside" Ethernet ports on the Zyxel so it is effectively just another device on your LAN. Disable DHCP which you have done, to stop it dishing out incorrect addresses. Also change the Zyxel's own IP address to be on the same subnet as your home router, but a different host address - this isn't strictly needed to get the wireless AP function working, but if you don't then you won't be able to manage the Zyxel from inside your network.