Please make sure that the permission of reading heart rate in Apple Health app.
Please see Can not display / record workout for access permission.
There is a setting of Workout Power Saving Mode in Apple Watch app > My Watch > General.
If the Workout Power Saving Mode is ON, heart rate will not be measured. Please turn it off and try.
Zones reads your heart rate registered in Apple Watch's health (HealthKit).
Therefore, the external heart rate monitor needs to be connected to Apple Watch via Bluetooth.
Please check How to measure the heart rate.