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My name is Ugo.
Since you've done basic trouble shooting and your iPhone 6S is potentially 3 years old, I would suggest you have the Apple store diagnose your phone's battery and then get a replacement.
After doing some research on Apple battery issues, it seems that after Apple's recent trouble with alleged performance-degrading, they're offering a 50%+ discount on battery replacements : https://www.apple.com/iphone-battery-and-performance/
Please let me know if my advice solves your issue. Thanks.