Contrary to the iPhone 6P, which had Touch controller IC's on the logic board which had a tendency to die when the phone is repeatedly flexed (i.e. Touch Disease), the iPhone 6S series (6S / 6S+), has all of the digital logic to control touch functionality built into the screen assembly.
So unless there is a power problem (still possible), the most likely solution to your problem is to replace the screen assembly. If your phone is under warranty, then you should return it to Apple for repair. If it is not, then you should look up some well reviewed, local third-party repair shops. they can put a new screen in to see if that solves your problem.
If you want to try it yourself, the you can get a replacement screen [product|IF315-037|here] and a repair guide [guide|55423|here].