You are mixing up the two things. Auto fan switch is separate circuit and thermostat valve is separate flow control setup.
The fan switching on automatically is due to temperate controlled switch which closes the circuit when coolant temperature crosses a set point, say 89 deg. C.
The thermostat valve is a valve which start to allow coolant flow towards radiator when the coolant temperature crosses a set point, say 82 deg. C.
I would say again, check for presence of thermostat valve, and/or its proper functioning.