iPhone 6 hasn’t turned on for years and I’ve tried everything

A couple of years ago, my iPhone completely died - it just did not turn on anymore. I tried charging it, holding the power button, holding the home and power button, but nothing worked. Eventually I took it to the Apple Store but they just told me that they can’t figure out what’s wrong with it so my only option would be to buy a new phone but this time with Apple Care. I bought a new phone and put the dead one aside, but recently found it again. I’m still determined to get the phone working and figure out what’s wrong with it. Today, I put in a whole new battery but still no luck. Does anyone have any idea what could be wrong with my phone or any suggestions?? Any input would be appreciated!!