When reading the Radxa docs they mention the following:
Thermal governor
This module is used to adjust the fan and other temperature options. Generally, we select power_allocator when using DC fan or fanless, select step_wise when using PWM fan. Note: If you connect the fan directly to the GPIO on 5V and GND pins, it would not be controlled and always work.
And according to support it is possible to connect a non-pwm fan to the fan connector on the board an control it to turn on and off when it hits a specific temperature.
I got a 5V fan (2-pin), which is connected to the fan connector on the board. How do I setup the temperature control in DietPi?