There is a strong possibility that the battery has failed in that iPad.
I would not even bother looking into a battery replacement, unless you can find a local third party repair center that can actually and competently perform a cheap battery replacement on Apple iPads, as this iPad model is nearly 7-years old now and not worth doing any type of repairs at this point in time.
Time for a new, new refurbished or much “newer” ( read used ) iPad.
If you must purchase older, used iPad models, DO NOT look at anything older than a 2014 iPad Air 2 or 2015 iPad Mini 4.
Both of these models are the oldest iPad models that the up and coming iPadOS 14 will support, in the Fall of 2020.
If you must purchased used devices from unknown, untrusted sources, you must ask A WHOLE LOT more questions of the iOS device seller, ask for more photo/images, ask if the iPad has user/iCloud activation lock and ask for photos/images/short video proof, ANY POSITIVE, UNQUESTIONABLE, VISUAL PROOF, that such an iPad has been reset to factory settings and shown proof of the iOS device's start up screen showing the animated, moving, multi-language “Hello” Welcome to iPad screen.
If the iOS device seller is unwilling to do all of this to show you that the iPad, in question, is ready to use with no barriers to new user activation and use, pass on the iOS device and look at other iOS device sellers, elsewhere, who will be more accommodating to answering questions and providing visual image or video proof of a proper factory reset iPad.
Sorry & Best of Luck to You!
