Set a DHCP range for the DGL 4500, e.g., 192.168.0.100-192.168.0.254. Set a different DHCP range for the EBR 2300 router, e.g., 192.168.1.100-192.168.1.254. Assign a fixed IP address to the Internet port on the EBR 2300 (192.168.0.2) and connect it to the DGL 4500. Then routing should work properly. Since the EBR 2300 is not connected to the modem (Internet), disable it's firewall.