Setting up dd-wrt as switch and dhcp for wifi?

Question

Network administration

There is a DIR-300 router, stitched on dd-wrt. There is a task:
1. Computers connected to LAN ports have static addresses, DHCP is disabled for LAN ports. These computers have access to the internal network, which is behind the WAN.
2. DHCP is enabled on the wifi port. Wifi computers have access to the internal network, which is behind the WAN.
3. An Internet cable is connected to the WAN port, static ip.
Can anyone suggest how this can be done?