On the external audio effect, Audio To and Audio From should both be analog 3-4. This is assuming that the physical inputs/outputs 3-4 on your interface are connected to the H9000’s analog inputs/outputs.
Don’t set the Audio To of the external audio effect to 1/2. This is creating a feedback loop (the master audio is sent out, and then returned to the input, sent out again, etc.)
If you playback audio on the audio track that you are using the external audio effect on, it should be sent to outputs 3/4.