Heat in top-right corner after screen replacement.

Hi,

I have recently replaced the screen of my iPhone 6 with a cheap replacement part from AliExpress (it had good reviews). First thing I noticed is that the phone is now a bit thicker but in a shop I was explained that this can happen with replacement parts (I wasn’t aware of that). No biggie, I can live with that. More problematic is that the battery drains super quick (it barely charges while switched on) and the top-right corner of the device (looking from the front) becomes very hot. I read in another thread that it could be a problem with a flex cable so I went to a shop to get it checked. They confirmed the problem and supposedly replaced the flex cable but the phone still gets very hot and so they said they can’t help me because it could be a broken mainboard (according to them). Now I have this phone lying around and I don’t know how to proceed. What would you suggest me? Are there ways to identify what exactly the problem could be? Any help is appreciated.

Thanks!