To manually set fan speed, first disable the automatic fan speed control by going into configuration mode and entering the sensor fanspeed- control off command. Write the configuration if it is to be retained across router reloads.
Now that automatic fan speed control is disabled, the fan speed can be set to any drive level from 50-100% with the user command:
test mbus set-blower-speed percentage
For example, the test mbus set-blower-speed 80 command would set the blower speed to 80% of full power.