This may be due to the graphics card on the one hand and the cable used on the other. How did you connect the screen? It must be a display port 1.2 cable to be able to transmit the required resolution.
I have an Nvidia GTX 1070 as my graphics card and use this cable here:
Club 3D DisplayPort Cable Version 1.2 - 21.6Gbps (3m, Midrange, Black)
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