iPhone 6s plus will not power on or charge

The charge circuit basically boils down to the charger & cable, the Lightning port, the battery and the various components involved in the charge circuit on the logic board.

Are you using an Apple original or MFi certified charger and Lightning cable? If not, then try that first. If that doesn’t solve the problem and you have already tried a new Lightning connector, then use a battery utility, such as coconutBattery (for Mac) or 3uTools (for Windows). Once you get down to ~70% of design capacity, you’ve pretty much used up the best part of the battery’s lifespan and you should consider replacing it. If you have really low design capacity, that may be impeding the charge altogether.

If you have access to a USB Ammeter, you could use that to determine if the phone is really drawing current when it says it's charging.

Finally, if it doesn’t appear to be charger, battery or Lightning port related, then you are left with a logic board issue and you should have an experienced micro-solderer look at your phone.