Using ethernet for iMac and Wi-Fi for other devices
Usually it's no problem to use the ethernet interface instead of the Wi-Fi interface, if the router/access point contains one or several ethernet connectors:


Install an ethernet cable between your iMac and the router. Depending on your router you might have to enable/configure its ethernet interfaces.
Open System Preferences -> Network.
Configure IPv4 of the ethernet interface by choosing DHCP or Manually. If you've chosen "Manually" enter an appropriate IPv4 address, netmask, gateway and DNS-server
Click on the gear in the lower left corner -> Set Service Order..., drag the ethernet interface to the top of the list and click OK.
Optionally you may deactivate Wi-Fi by highlighting it in the left pane and then "Turn Wi-Fi off"
Then hit Apply