QGC not discovering remote compass

I have a Holybro Pixhawk 4 running PX4 Pro 1.11.3. I am using their GPS-compass-switch-buzzer combo. When I look at orientations in QGC, all I see is the internal mag, which I have set at yaw 180 deg. The external mag is not showing up, and I have to set it to 0 degrees. Since the internal and external mags are 180 degrees apart, how do I calibrate this? Comparing the QGC compass to an external compass, the two readings are far apart.

How do I tell QGC that the Pixhawk and external compass are 180n deg apart?