Maximum temperature of an electric motor and proper cooling system
That is not a motor

That is an electric linear actuator.  Those are rather pricey/expensive beasts in the €300 range, and yours is a quality make.  So be gentle to it and don't destroy it. 

If it's rotating the shaft, that's because the other end of the shaft is supposed to be held from rotating!   At that point, you will see: the shaft will move in and out.  

It will never work as a motor.  If you tried, the moment it hit any rotational resistance, it would start moving in/out like it's supposed to.   Yes, you removed the limit stop, but that means it will spin continuously at the limit stop.  The bearings are not designed for that, and it will quickly wear out. 

The motor is not designed to run continuously.  It's not designed to run any longer than is required for the motor to run its travel. 

The label states the duty cycle.

No more than 30 seconds on, then at least 120 seconds off. 

Get the right thing for what you need.

I don't see any way to cool this thing enough for the duty cycle you want. Peltiers don't work that well; they make far more heat than they move.   

You probably want a gear-motor. You could sell this unit for enough to get a couple of gear motors.  Those can be had rated for continuous operation.