Trainer Difficulty Disabled

Hi. I have Zwift on my Android phone, and an Elite Zumo trainer. I used to cycle round Watatopia mostly but switched to training with ERG mode on. Since doing this, my trainer doesn’t respond to gradient, and tellingly, the trainer difficulty is disabled. I’m aware that in android you can’t do any trainer settings. Is this where I might be able to fix this? I unfortunately don’t have a PC or Mac (only a chromebook). Thanks!