Why is my ipad screen not responding to touch?

Hi i just bought my ipad air 4th gen through a third party app (it’s second hand but the condition is brand new). So i just used it for 4 days and all of a sudden after charging it, the screen doesn’t respond to touch but the screen display is fine. I just can’t get it to respond to touch. I tried force restart, factory reset, nothing worked. I send it to a repair shop and they said the motherboard was fried due to overpowering and they managed to get it fixed (im not entirely sure what they had done but my ipad worked just fine after). A month later, the problem resurfaced. I sent it back to them and they said it’s the same damage so they repaired it again. The ipad is okay now. However, I discovered that there’s an invisible line (about half inch thick) on the screen where I can’t draw on. I tried drawing using the apple pen and also my finger, both won’t work. That part of the screen is responding to touch, but i just can’t draw on that part. I don’t know what to do and I can’t keep on wasting my money on repairs when it’s just gonna cause more problems. Please help. Thank you🙏🏻