Pressing TO/GA (buttons on side of thrust levers) is applicable when ATS is not active on the ground. What you are actually doing is waking up the flight directors and giving them lateral and vertical modes (TO/TO). The arming of ATS to N1 TO (or FLX or MTO, if selected) is more of a convenience.
So yes, on the ground, if you want to “reset ATS” deselecting the FDs and pressing TO/GA is a way to do it by virtue of resetting the automation entirely.
Other times, say ATS is in FAIL, the way to “reset” it is to press the AT disconnect buttons (front side of thrust levers). Or if the ATS is off in flight, select a valid N1 thrust limit on the CDU and press the ATS button.
On the topic of missing N1 limits, suggest this thread: