First Alert Smoke/CO gen 2 won't connect

I get to the stage that says First Alert Smoke Alarm. Slide open the tray, insert the batteries and push in the tray. After doing that, it beeps, but nothing happens in the app. I've spent the last two hours reading and trying various reset remedies. This is to replace a defective unit, so now I've had 4 units, and 2 seem defective/won't pair.