Brand new iphone 6s Plus

“Brand new” and “iPhone 6S Plus” do not go together very well, as the iPhone 6S Plus was introduced by Apple in 2015 and discontinued in 2018.

If your iPhone, iPad, or iPod touch won't turn on or is frozen - Apple Support

Well, there’s certainly something fishy about this scenario. Again, Apple discontinued selling this device about a year and a half ago.

Did you purchase this at an Apple Store or Authorized Apple Retailer?

Was the price “too good to be true”?

Instead of being ready to give up on iPhone, maybe buy a current model, not one discontinued long ago, which is also very likely in its last year of support, meaning when iOS 14 is released this coming fall, it won't be able to even run it. The chances are also that the iPhone 6s you purchased, whether it came in a new box or not has no valid warranty from Apple today. Amazon is not an Authorized Apple Seller, so whichever seller sold it, would have to offer a buyers warranty as the warranty on that phone started the day it left Apple's inventory, not the day you open the box.

Most likely the battery failed. It would be rare for the device itself to fail in storage, but batteries are another matter. It should be eligible for service, although I’m not sure how they handle a DOA device that has never been activated.

I would suggest bringing it in for service. Most likely a failed battery that can be replaced for Apple’s relatively low price.

Brand new?

Apple discontinued iPhone 6s long ago.

Bring it back for a refund.

Sorry, in the box means nothing.

Return it for a refund.