Won't power on , instead it automatically powers on

So here's the issue.

1. 6 months ago the screen of the phone broke & yesterday i got it fixed through unauthorised channels, but when i connect it to power outlet (not the original charger) it automatically powers on while plugged in & encryption password prompt appears (which is forgot), instead of showing charging screen & powers off after plugging out showing 0% charge.

2. Since the phone automatically powers on i thought of hard resetting it to factory setting but was unable to do so because when i select the factory reset setting by pressing power button , the phone would turn off automatically.

So in a nutshell phone is not charging after being plugged in for complete 1 day & unable to hard factory reset it.