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Plug into the Ethernet network.
Make sure Wi-Fi is on and connected a Wi-Fi network.
Open System Preferences.
Select the Network Preference pane. It will look something like this.
You will note that this Mac has a built-in ethernet connection. Your laptop will probably have “USB Ethernet” if you use a USB adapter between the ethernet cable and the laptop.
Below the list of possible connection methods, there is a “gear wheel” menu. Click on it and choose “Set Service Order…” as shown.
In the Service Order dialogue, drag the connections up or down until you have Ethernet at the top, and Wi-Fi next in the list.
Click “OK” when done.