Once the connections is established QGroundControl creates a popup that says
Parameter are missing from firmware. You maybe running a version of a firmware that is not fully supported or your firmware has a bug in it. Missing params: 1:FLTMODE6, 1:FLTMODE5, 1:FLTMODE4, 1:FLTMODE3, 1:FLTMODE2, 1:FLTMODE1
Why does QGroundControl expect these and how do I tell it that it should not. Those existed in ArduSub V3.4 but not anymore.