Replaced logic board did not solve my problems

So I recently purchased a used early 2015 macbook pro A1502 (for a really good price) which worked for a few days until one morning it just didn't switch on. So the few things that seemed to be an issue with it was that when I connect the magsafe adapter to the laptop there is no light coming on the adapter and the laptop would not switch on. I tried again but this time holding the power button as I plugged it in and it turned on with apple chime and full fan speed but no display.

What I did was point a flashlight through the lid and noticed that it was infact displaying stuff just that there is no backlight (also no keyboard backlight).

So I plugged it to an external monitor and I was able to log in and do stuff.

Next thing I did was open up the back of the laptop and have a look at the board and to my surprise it was a absolute mess! Seems like the previous owner might have tried to fix some issue but did a horrible job. I noticed that the DC in board had really rough solder marks on it, some logic board screws were missing and worst of all I do not know why anyone would solder the camera cable to the jack on the logic board together!!!?

So I have a laptop with:

1. No display backlight
2. no keyboard backlight
3. does not switch on properly
4. fans running full speed
5. no battery detected

So I replaced the logic board thinking that would most likely solve my problems but guess what? It didn't.

My thought were that if the laptop could switch on the DC in board couldn't have been the problem but now that it doesn't. Could it possibly be the culprit of all those issues I'm having?

Thanks in advance for any input.