Audio issue in hybrid meetings

A have a hybrid class with some members f-2-f and some participating via Zoom. The audio for the Zoomers is variable unless the f-2-f members sit close together near the computer mic. I have bought an external mic with an extension chord that plugs into the USB slot on the class laptop (with large screen monitor),  I placed it  in the centre of the room with tables pushed close together, but it does not activate as the computer mic overrides it.  When I set up the meeting, the first dialogue box that appears is "Join with Computer Audio" which suggests that other options are available. I have tried the Windows "Settings (audio) but there appears to be no way to switch to the External microphone. What do you suggest?

Joe