iPhone X not turning on anymore

I bought an used Iphone X that had broken back glass for me to try and repair the backglass, as a bit of an practice.

The phone when i bought it and tried it first did not seem to turn on, then with a second attempt the screen flashed a bit before it eventually stabilized and was turned on. I found out that the device was icloudlocked and turned it off as the seller said that he could fix it, but now after charging with the exact same charger and cable as i did when the phone was alive for a bit for 7-8 hours, the phone seems to be dead.

The charging port seems fine, the only broken thing that i know of is the backglass, so what could be the issue/how could i solve it possibly?