When using a device with both Ethernet and Wi-Fi such as a laptop Windows will by default favour the Ethernet over Wi-fi. This means that if the laptop is connected wirelessly and you plug in an Ethernet cable it will immediately stop using wireless and switch over to using Ethernet
For some reason it is possible that this transition will not occur and it might still favour the Wireless connection.
Here is a way to make sure it uses Ethernet
You can also verify which connection Ethernet or Wi-Fi is going to be used by opening up Command Prompt and typing route print
Then look for the gateways of your two networks and see which one has the lower metric that is the connection that will be used.