Using Edge 520 with Training Peaks workout on Turbo

Help please?! I have recently bought an Elite Justo trainer and am trying to do the ramp test which is in Training Peaks. Training Peaks has synced fine with my Garmin Connect account and shows up on my Garmin device (Edge 520) to follow the target power each minute. However... the issue I have is that when I increase my leg speed at the start of each minute (to create more power as required by the test), the trainer lowers the resistance making it easier to pedal and my power output therefore drops without me wanting it to. It is as though the trainer is somehow being controlled by the Garmin? Erg mode kicking in somehow? (which I don't want) Is there a way to stop this happening? Many thanks for any help with this - I'm getting so frustrated with trying to find a solution!