Dead Iphone 5s

[oberlander]

Hello everyone I'm new of the forum so that's my first post, I'm writing this to have some advice regading an used Iphone 5s that i recently brought from the bay as broken. Seller told me that it was dropped and it has no power now. Visually inspecting the casing looks good except for the upper rear part, near camera flash where the glass is shattered to pieces. Screen looks good, casing is not twisted. Upon receiving it I did as following: Tried to have power by pressing power button (obviously),plugged it to a wall outlet, to a macbook laptop and a windows home pc to get life on it, tried hard reset (power+home buttons) while keeping it connected for more than some minutes. Still had no power after that so I opened the phone, the internals looks good and intact (seems never been opened before) with no apparent damage, no sign of corrosion or whatever so I had disconnected battery from motherboard and after that plugged to the computer to see if was that the problem, didn't have any sign of life. Lastly I completely took apart the phone pulling motherboard out and reinstalling it with only the charging cable connected and no screws mounted, had no joy though. Note that I'm using a cheap aftermaket cable (that did well while testing an iphone 5 for a friend) while in one occasion macbook showed an odd warning message like "some usb device is draining too much power, usb functionality is temporarily unavailable" with only the seemingly dead phone connected to it and sometimes touching the lead i feel it slightly warm. So that's the story, I'm thinking that could not be the battery the fault, could it be the infamous power ic that went bad?. I'd rather buy a geniune apple usb cord and retry all the steps or try change the phone's charging port with a new one. That seems reasonable?. What would be the best bet?. Thanks to you all in advance!