If you already connected via Wi-Fi first, then connected the Ethernet cable for a stable connection and want the Hotspot only works via Ethernet, you will have to remove and forget the current Wi-Fi.
Unplug your Ethernet cable and finish the steps below. You may reconnect the Ethernet cable again once finished.
Open Helium App
Pair your Hotspot by pressing the button on the back of the device for 5-6 seconds.