Dell D3100 - no 4K resolution available

[gdevans]

I have the following equipment:
Dell Inspiron 17 laptop - Windows 10
Dell D3100 dock
CableCreation DisplayPort to HDMI adaptor (supports 4K at 30Hz)
QED Performance eFlex HDMI cable (supports 4K)
Sony KD-55X8005C 55" 4K TV

I installed the latest driver on my laptop, and connected the equipment. Everything appears to be working correctly. I get a good display on the TV.

However, the maximum resolution is 2048x1152. This resolution is not compatible with my display, but 1920x1080 works correctly.

I wish to run it as a UHD display at 3840x2160. I checked the Adaptor settings, then "List all modes". This does not display the 3840x2160 resolution either.

Please note that I am connected via the DisplayPort socket. I am using a DP to HDMI adaptor which is 4K compatible. I am not connected to an HDMI socket.

Does anybody have any suggestions please?