I have a older Dingo 150. It stalled one day and would not start. The starter spins but the engin does not crank. I replaced the starter, same thing. I took the cover off the right side; looking forward from the driver seat. The mag turns fine and it rotates the crank. Not sure if the gear just inside the housing the started inserts into is spinning or what, but no crank.

 

Your starter clutch is slipping. This a one way "sprag clutch" (google that for more info) that allows the starter to turn the engine, but when the quad starts up it freewheels to keep the engine from spinning up the starter motor.

Is your engine a GY6 motor? There are full blown (and free) service manuals available for GY6 motors.