Need Help - iPhone 4 not turning on?

My Iphone 4 is water-damaged and is not turning on? Have tried plugging it in to pc / power socket / hard reset by holding home/power button for seconds - without any success. Screen doesn't light up, no sounds is heard.

I can confirm the following is working as I have a Test phone with all working components...

1. Water-damaged logic board of iphone IS working as normal (confirmed by removing the water-damaged logic board and trialed it in my Test phone metal framework - everything was working).

2. LCD screen & digitizer is a new replacement and confirmed as working

3. Battery is new replacement & confirmed as working.

4. Power button & home button & charging dock - I think it is working??? Based on plugging water-damage dock into pc, then putting the water-damage iphone into DFU mode. The pc detects this in DFU mode. Thus, the power button & home button should be working - as able to go to DFU. Also the dock connector should be working based on pc able to detect phone when in DFU mode. Seems logical reasoning?

So now I am lost as to why my iphone is not turning on - what other problem can it be?