Newly installed thermostatic mixer runs cold when shower head raised in height
The height of the cylinder is immaterial. Is the hot water from a loft tank or the mains pressure supply?
In most cases the hot and cold must be from the same supply or at the same pressure for a mixing valve to operate successfully. You can boost the hot with a pump or raise the height of the header tank, use a pressure reducing valve on the cold to reduce it to closer to the hot, or use a mixing valve which will tolerate differing supply pressures.