What part would stop OPPO phone from booting up besides flat Battery?

I flashed my Chinese OPPO with Bacon's 44s and it was fine for a few weeks and handling good battery life as well. One day it just switched off and I have tried 5 x 20 second battery cycle presses, Fastboot mode, recovery mode and USB cable to PC. None of these options are showing any lights, vibration or any sign of life. I ordered a genuine battery and just like the original it too gets warm when charging but still no boot up. Please note this phone has been well looked after and never been dropped.

So if the Battery is fine then what else could it be?

Other than Ali-express I found Vifocal.com who I ordered a replacement battery also supplies all the parts for the OPPO.

Any help from experienced Android repairers would be greatly appreciated.