In order to exercise in the right zone, you are going to need an accurate heart rate monitor. The Garmin HRM-Pro is a premium heart rate monitor chest strap with Bluetooth and ANT+ connectivity. It is designed to work with Zwift and other similar fitness apps. It captures your running dynamics to help you become a better runner.
This heart rate monitor can store heart rate data with your watch is out of range. It runs on a coin cell battery that lasts up to a year. This heart rate monitor stores steps, calories, and intensity. You can find out more about it here.