I don’t own a Hydra, but looking at the manual it seems that it can only send Program Change messages 0-7.
This means that if you connect it to the H90, you’ll only be able to switch between programs 1-8.
To do that go to the System menu (hold the Program and Routing buttons), go to MIDI and change PC Offset to 1.
Also make sure that the Hydra is set to MIDI channel 1.
