|You currently have two routers in your setup and both are doing DHCP. That is why the two systems connected to the Leviton cannot communicate with devices connected to the Airport. They are in different IP subnets.|
I have not read the Leviton manual nor the Airport but if you want all devices in the same subnet then you should disable DHCP in the downstream router ( The Leviton ) and connect to it's LAN port rather than WAN port, or discontinue using it anyway since it is only a 10/100 device.
Replace this with a gigabit switch.
You have not really defined your proposed network. Do you intend or already have Cat5 cable run from Modem/router location to each user location in the home or do you intend to use wireless distribution , and then use Wireless Bridge devices & Switches share wired connections in the various rooms ?
Goin' Fishin' (Some day)