None of the youtube instructions work, I asked DJI support, the lady's reply get me more confused, I doubt she really knows the answer. Is there any clear instruction for sport mode under new firmware?

Hi, there. Sorry for the unpleasant experience caused. To cover more shooting scenarios and requirements, the “Sport Mode” on the Gimbal Settings menu was changed to “Follow Speed” from DJI Mimo v1.6.4.
To enter the Sport Mode, set Follow Speed to be “Fast” or follow the old method by tapping and holding the Trigger. Hope this helps.