iPhone 6S charges, but won't turn on.

Hi,

I purchased an iPhone 6S registered as “for parts” off of eBay in the following condition: broken screen, dented on back and sides (such that the power button was lodged in) and not turning on.

I opened the device and inspected the general condition, and internally it was pristine. I completed a teardown and made it so the power button was clickable and replaced the battery with a new one (OEM quality).

Upon charging, the screen does not show anything up and I've tried turning silent on and off, etc and there are no vibrations. However, the device does warm slightly, suggesting that it is actually charging.

I've tried: holding the power button with the home button for fifteen seconds, holding the power button for three seconds and then holding the home button and tried connecting it to a computer.

I'm not sure what the problem is, as I'm not the most experienced with repairs and would appreciate any help. I also apologise in advance if my question is wrong or doesn't meet the community guidelines, as I'm a complete noob on iFixit.

Thanks,

Dan