My iPhone cannot turn on

It cannot turn on at all unless it's connected to a wall socket, and even so it would be stuck on a reboot loop. I put it in recovery mode but the screen goes back to black the moment I unplug my cable (to plug it into my computer).

What I've tried:

Hard reset

Restoring and updating via iTunes (many, many times)

Factory resetting and restarting as new iPhone

^ all these wouldn't actually work now since it can't turn on...

The problem started with it being in a reboot loop then it got better where I could use it normally as long as it's connected to a power source. But now it got worse and hard reset doesn't even work anymore.

I believe the problem may have arose because my phone was low on battery when I updated the iOS, although it was charging when it was updating. In fact, it was only until about 12 hours later that my phone started to have the problem.

Not sure if it helps but it's an iPhone 6s plus

Could it be a hardware problem :/?