Patur, the Tanna is simply saying not to work with the reward in mind. Of course, ??? ???? ??? ???? ???? ??????. You are doing the Mitzva anyhow, and you were originally motivated by reward. But while doing it don’t make the reward your focus.
You should strive to love Hashem and His Mitzvos to the extent that you’ll yearn to do them regardless of their reward. And yes, your motivation for making yourself this way will be for your benefit. But the level you will have arrived at will be about not considering reward.