The router will also consist of a network switch. You're just using it as a network switch, without any of it's routing. https://en.wikipedia.org/wiki/Network_switch
When you buy a commercial router, you're getting several devices in one, often a modem, router, network switch, wireless access point and possibly others depending on the model.
By connecting to a LAN port and disabling DHCP, you're only using the network switch element of the device, as in, you could be using a network switch to do the exact same job. The DHCP and NAT routing now comes from your only DHCP and NAT enabled device on the network, your WLAN router. You've effectively just added more ports to your current network.