Charging problem and boot loop

I have a iPhone 6s plus that I've had for ahwile that my uncle game me to fix so I order a brand new screen and I plugged the phone it a charger and when i start it up it's starts a boot loop so I've had my experiences with bootloops and know how to get around them so I unplug the charger to hard reset the phone and the phone instantly turns off . It only turns on when it's connected to the charger and goes to the boot loop so I connected it to a computer to restore it and when it easier goes to restore it the phone restarts 3 times and I get an error on iTunes saying there was an error trying to restore iPhone also it doesn't get to the load bar thing it just keeps restarting to the plugged in to iTunes prompt on the phone. So do I have to buy a new battery or replace the charging port?