iPhone is native Bluetooth (BLE). Your HR strap is quite likely ANT+.
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You can get a new HR strap that does BLE (and ANT+ for most options in the future). This is about the easiest option.
- The Wahoo Tick, Bontrager HR and 4iiii Viiiia are a few options.
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You can also look into getting an ANT+ dongle for the phone. I am not an Apple expert, but know there can be some issues and limitations with this direction.
- This have been covered before, but I’m having trouble tracking down the prior discussions at the moment.