I researched a little and found that Windows 10 already prioritizes ethernet over wifi (I didn't even know Windows 10 put ethernet first by default, I haven't had this issue since Windows 8).
So if you are running Win 10 then it's already using ethernet as the primary. At this point, I would suggest disabling ethernet or wifi. I'm not sure which method you tried to switch off your wifi, so I'd try this method in case you haven't:
Go to Control Panel > Network and Sharing Center > Change Adapter Settings > Right click on the wireless adapter > Hit Disable