How do I get a Garmin Cadence Sensor 2 to pair with an iPhone?
It seems like it could work using their Garmin Connect App.
See Page 5 below.
https://www8.garmin.com/manuals/webhelp/cadencespeedsensors2/EN-US/Cadence_Speed_Sensors_2_OM_EN-US.pdf
See this snippet too:
https://www8.garmin.com/manuals/webhelp/cadencespeedsensors2/EN-US/GUID-E87E78BB-03DD-48C1-AA67-A69E591B01FC.html
However, I think optimally this thing is designed to be used with a bike computer Garmin Head Unit vs. a phone as I am not sure how it will display the cadence info as you ride. Note I never used my phone as a bike computer so perhaps there is a way to do that.