Using a webcam with a Macbook Pro

[Bad_Robot]

I have a paid account and use Zoom for regular meetings. I use a Macbook Pro with an external monitor and I bought a Logitech webcam to sit on the monitor to use instead of the MBP built in webcam.

I launch the meeting and the Logitech is selected automatically, but then after a few minutes, the webcam will shutdown and I get a message saying that the default camera/mic  has switched to the MBP webcam/mic. Video goes blank, audio goes dead.

I reselect the webcam, few minutes later, same thing happens.

I don't want to use the MBP with the lid open, I want to use the external monitor.

The Zoom app will randomly decide it doesn't want to use the wecam, then because the laptop lid is closed, it will switch off video entirely.

The other strange thing the app does, if I have my iPhone near, I can select the iPhone camera and mic, but the app will stay on the iPhone and never switch off or try to switch to the MBP.