iPhone won’t charge.

Hello, I’ve been having can trouble with my iPhone 6S. It will hardly charge. I will leave it on the charger and come back to it being 100%. As soon as I unplug it, the phone will turn itself off. When I turn it back on it’s all the way down to 10%. I’ve tried restarting it and it just keeps turning off and going back down to 10%. It dies within 20 mins of using it. There’s no cracks and no damage at all. It’s completely updated and has storage. I wasn’t sure if I should call my provider, T-Mobile or Apple Support.