iPhone 6 won't charge past 3%

I got my IPhone 6 second hand from a company called Quick Mobile Fix. Within around 4 months of having the phone it started staying on a percentage (e.g. 36%) for a few hours then suddenly dying. I ignored this problem and did nothing. However a few weeks ago it did the same thing but didn't charge or turn back on when I plugged it in. After about two days in the charger it turned back on but would not charge past 3%. I then sent it off to a company for it to be fixed. They sent it back saying they had fixed it however it still does not charge past 3% and when out of the charger it suddenly dies. Not sure what to do next - would be grateful for any suggestions..

I have tried resets and things like that btw :)