iPhone 6s after screen replacement boots up then dies?

After replacing my faulty iphone screen with the correct screen, and checking all screws are correct and all cables were replaced exactly, I tried to power up my device.

What happened was that it powered to the boot logo for about 5-10 seconds and then completely went blank. Then I tried plugging in a cable to the wall - it made the noise. Then I thought I placed the lcd wrong so I re-checked the cabling etc. and then rebooted. Next, it just displayed the 'plug in charger - out of battery' logo. So I plugged it in. Then (every time I powered up my device it showed the same logo) it just died mid logo...

Then I read online that I may have to give it time to do something ( I'm confused by this) so ,having no other way to power it down, I disconnected the battery cable in the phone. Then I waited and powered on and it just repeated the out of charge logo situation over and over and (I kid you not) over again -- no sound of charging anymore (I changed cables as well).

Next I tried to connect to itunes and then it recognised (yay?) but told me to unlock the phone... but I had no way to actually unlock the phone..

Now the power button seemed to not affect the power down(?) so the only way I could reboot was unplugging the battery cable. So that's the only way to display logo once before it died. Even now, plugged into the computer, it doesn't power on or show any signs of being alive...

So then I read it may be caused due to battery but I am wondering can a faulty battery (or even dead) be recognised by itunes?

P.S My screen didn't die as it did show the logos (idk about backlights)