So you have a router meant for a cable connection that you want to use as a simple access point on a network which already has it's own DHCP and DNS.
Here's what you need to do.
1, Configure wireless security and SSID.
2, Turn off DHCP.
3, Connect your network to one of the LAN ports, not the WAN port.
If you connect to the WAN port the Wi-Fi connection will be behind NAT whereas the LAN ports are basically on a switch connected directly to the Wi-Fi.
