Nia Kobo Nia touch screen troubleshooting

[corank]

Hi everyone!
I got a faulty Kobo Nia for free. Actually everything else seems to be working fine except for the touch screen. The symptoms include:

Most of time it does not respond to touch. Occasionally it does but that only lasts a few seconds before it stops responding again.

Even when it responds, it does not appear to report the correct position of touch.

Everything else is fine. The device reacts can sleep and power off/on. I also reset and upgraded the firmware and installed Nickel Menu, KOReader and Plato without encountering any problem. The issue with the touch screen remains the same throughout.

I removed the back case and tried reconnecting the touch screen pins but I'm not sure it helped.

I don't have much experience troubleshooting or repairing hardware. Is there anything else that I could do? Thank you!