@Mittie 

Test the live video on other phones to see if the issue is just with that phone. What you can try to do is to disable Hardware Acceleration, but that needs you to be able to get into the camera settings. If the camera is greyed out, delete the camera. If that is actually the camera, then the LED light will change. If the LED stays green, then that is not the camera, could be another camera.

If that is the only camera, do a factory reset on the device by holding the reset for 20 seconds and then release the reset button. Once the camera comes back up, add it back to Kasa, if the live video is still dark, go into the camera settings, scroll all the way to the bottom to "Advanced Settings" and disable "Hardware Acceleration"