touch screen/lcd not responding

I had got the LCD screen replaced a couple of months back after the screen broke from a fall. It worked fine until I upgraded to ios 11.3.1. last week. Immediately after the upgrade the screen started acting up - would respond for 5 mins and then go completely unreponsive. After 20-30mins, again would respond to touch for a few mins and then nothing.

I to reset to factory setting using itunes, restored from a backup in itunes. And after that now it is completely non-responsive. I see the screen but can't interact at all with touch.

Took it to the same repair shop, they tried to put a new lcd touch screen but said even a new one is not working.

No idea what else could be wrong? Any help/suggestions would be appreciated!

Thanks