|If your server should run as DHCP server, non of the routers should run DHCP to avoid trouble.|
The routers can be connected simply by using an ordinary network cable from LAN port to LAN port.
You can use the wireless ports as switch ports, because the wireless router has a switch just like a non wireless router.
As mentioned above, on the ISPs router, you also have to disable DHCP.
If you'd like to use the WAN port of the wireless router to connect to the ISPs router, you have to configure the WAN port for static IP. The IP has to be in the same range as the ISPs local IP address.
In case of using the WAN port to connect to a LAN port of ISPs router, the wireless router will secure the internal network with it's firewall.
The routers firewall secures the traffic from the WAN port to the LAN ports but not from LAN port to LAN port.
Please send a reply, if you solved the problem !!!