I guess the precursor question is: Is the camera working at all?
This is of particular significance with the Librem 14 (and other Purism devices) because there is a physical switch that disables the camera. So have you checked the switch to make sure that the camera is on?
The problem with this question is that in reality in many situations you do not control the choice of meeting platform. Let’s say in a work scenario someone organises a video meeting for a cast of thousands, they aren’t going to change to a more open source-friendly or a more privacy-friendly platform just for you.