@mcbiker, you might read this forum discussion Chainring and Turbo Trainer which talks about ERG versus level mode. I’m new to ERG mode too and it was really jarring to try to get the gearing right. The video in there is really helpful too.
To summarize, in ERG mode: use a high-ish gear and don’t change during the workout, spin your cadence up a bit just before starting a harder interval and slow it down just before a recovery interval.