The Garmin Cadence Sensor 2 will not pair with an iPhone via the Garmin Connect app, nor directly from iOS Bluetooth Settings. You need to use an entirely different app—I have successfully paired it via Cadence and the Wahoo iOS app.
It seems like it could work using their Garmin Connect App.
See Page 5 below.
See this snippet too:
However, I think optimally this thing is ...