Charging problem followingg screen replacement and water damage

My son dropped his phone into a swimming pool last week - not happy! The front screen and back cover were already cracked so we stripped it down to replace those whilst drying out and cleaning the rest of the phone. It works great - all the apps do what they are meant to do, takes photos etc.etc. BUT it won’t charge. I have changed the battery and USB charging port (and charging cables obviously) but still get a white battery icon which has now dropped down to 30% from 60% when first connected. I have factory reset the phone several times but still the same.The only thing I didn’t change is the flex cable between the motherboard and the battery area (assuming that it was still operational because the phone works in every other sense). Could it be this cable or am I now wasting my time and having to admit defeat. The phone is about 18 months old. Thanks