One thing to keep in mind is that other apps or devices that have paired to your trainer or HRM via Bluetooth can hijack the signal if they are still running, so shutting off or deleting anything that has ever paired to the trainer via Bluetooth can help, and any of those devices paired to the phone’s Bluetooth service should be disconnected and “forgotten”. With ANT+ the read-only sensors such as power, cadence, and heart rate can generally be paired to multiple devices without harm, but there can be only one thing controlling resistance.
If you buy an ANT+ receiver, you may need the information in the thread linked below. The Coospo RC401 is an inexpensive ANT+ dongle that works fine and is widely used.