Phone turn on just when plugged in

Hi guys, im facing a problem with my M7 a couple months, it only powers on when charging, tried to charge it for a entire day, but doesnt work. Tried the volume down + volume up + power keys for 2 min, and other stuff, but none of it have worked.

Everytime i turn on the battery shows a different value. if i plug off the wall charger, the phone suddenly shuts down.

What can it be?

Thank you!