This happened to my 6s. Tried several cords, squares, outlets, same problem. Took the phone to Apple, they checked it over and decided that several pins inside the charging port were damaged and the only option left was to trade in the phone for a new one. So 30 minutes and $250 later, I walked out with a new iPhone 7. My advice, don’t try to clean the port yourself. You will just end up damaging those fragile pins inside the port. Take it somewhere where a professional will clean it.