To get distinct subnet ip addresses for your wifi devices from your lan devices, if you have multiple wifi routers you should connect them to seperate hub or switch connecting it to one of interfaces of your DHCP server say eth1
Other interface say eth0 will be connected to the switch which is having all lan device connected to it.
Now Lets start with configuration,
When a DHCP configured PC boots, it requests its IP address from the DHCP server. It does this by sending a standardized DHCP broadcast request packet to the DHCP server with a source IP address of 255.255.255.255.
If your DHCP server has more than one interface, you have to add a route for this 255.255.255.255 address so that it knows the interface on which to send the reply; if not, it sends it to the default gateway.
Add the route to your /etc/network/interfaces file. In this case the route is added to the eth0 interface.
# File: /etc/network/interfaces
iface eth0 inet static
up route add -host 255.255.255.255 eth0
iface eth1 inet static
up route add -host 255.255.255.255 eth1
Now, You can configure your DHCP configuration as given by @2707974
Suppose request comes to say eth0, which is configured with address with subnet
192.168.0.X so that client will get ip in
Similarly, for eth1 interface.