Do you have a dual HR monitor? It may be that your HRM is not broadcasting on bluetooth. My old Garmin chest strap would only broadcast on ANT+. Your iPad will only be able to pick up Bluetooth signals. I don’t have an iPad but I don’t think you can just plug in a dongle. I don’t think apple supports ant+.
A way round it it would be to get a ant+ to BLE bridge, or get a HRM with dual Bluetooth and ANT+ output. Garmin do the dual model. I have the Polar H9 which does Bluetooth and ANT+ and broadcasts on both at the same time.