iPhone Xs Max original touchscreen does not work

I am not sure how the original problem occurred maybe a drop or some liquid damage. I don’t know what equipment you have on hand but I would separate the boards and install them into a test fixture, then connect an iBridge to the touch connector and boot the phone. This way you can completely rule out the touch circuits with voltage and diode measurements while the connector is in place. If everything measures correctly with this procedure I would be looking at two scenarios the first being driver software, it use to be quite common that an original screen would not work but an aftermarket one would on the XS series. Sometimes a DFU restore would fix the problem and other times it would not and would need to have apple calibrate it with their dongle. One of the other causes would be dirty data lines but I did find that the touch worked occasionally. You would need to use an oscilloscope for these measurements. Have fun down the rabbit hole.