A long lag in the ERG won’t help, but yes, you need to put it in a gear like that and don’t change gear. In ERG mode the trainer adjusts the resistance so that you’re generating the power target. If you change gears or pedal slower/faster then it has to adjust, and any lag of course just makes it take longer to adjust properly.
It’s counter-intuitive because you pedal faster and it needs to reduce resistance ro maintain the power target (since otherwise higher cadence = higher power). Similarly it needs to increase resistance if you pedal more slowly (which leads to a “death spiral” if you’re not careful and you pedal slower and slower with increasing resistance.
So yeah, middle ring at the back (or whatever gives you a nice straight chainline) and the small ring at the front if you have a choice there. Then just try and keep your cadence nice and smooth, and if you see the power is too low when you’re up to your desired cadence then don’t pedal faster, just wait for the trainer to catch up and increase resistance.