My new Payne heat pump condenser unit has frost around the coils. The defrost cycle does not seem to be coming on. Could it be a control wiring problem? And, is the defrost thermostat considered a outdoor thermostat?

They will have frost around the coils when in operation in cold weather but the unit will go into defrost when necessary if it is operating properly.

The defrost thermostat senses when it is time to defrost. Yours has a circuit board and a sensor.

A outdoor thermostat just shuts the outside unit off when the temperature gets to low for the unit to operate efficiently.