Hi @reimonha,
It may be necessary to add another UDP connection to avoid connection conflicts between QGC and your Pymavlink program - in general only one program can connect to a given port.
When the connection is established correctly it should be possible to control the simulated ROV with a joystick, just like normal. Note that there will likely be issues attempting to do so if you are sending commands via a separate pymavlink program at the same time.