How to synchronize the RGB Video?

I use orbbec sync hub pro, Femto bolt, and I refer to the website (Record Multi-Device Synchronized Data with Femto Bolt Recorder - ORBBEC - 3D Vision for a 3D World).
However the output video is not synchronized…

Is there a additional procedure to synchronize the output videos?

The command I used is
master : “k4arecorder.exe -d OFF --imu OFF --external-sync master -r 30 -l 60 out.mkv”
sub: “k4arecorder.exe -d OFF --imu OFF --external-sync sub -r 30 -l 60 out.mkv”

Please let me know if you know about this issue!