Quick charge 3.0 is already enabled on your phone! ZenFone 6 supports QC 3.0 at 15 watts, but we can only qualify that with the bundled ASUS AC adapter. Performance with other electrical sources may vary.
Try flipping the power adapter in the wall outlet by 180 degrees. Mine was charging slowly and after messing with all settings over an hour, I tried flipping the power adapter and lo! 'Fast charging' is back!