Why won't my iPhone 6s turn on or charge?

I have had my phone for nearly two years now, since December 2017, and six months after I got my phone it my started turning off and not charging. I have taken it too the store multiple times and they have replaced the battery every time but it is still completely dead and won’t turn on. I have tried simultaneously holding the ‘home’ button and the ‘power’ button but nothing works.