Dell XPS 13 and external display resolution

Hi, 

I recently purchased a ultra hd external display from dell as well as the D3100 dock to connect with my new XPS13. 

I have not been able to use the display via DisplayPort and have only been able to connect via HDMI with a resolution of 2048x1152.

According to Dell's website, the intel graphics card 5500, the xps 13, the d3100 dock and the monitor are all compatible and I should be able to connect via DP and get a higher resolution using HDMI.

I've checked updates and everything is up-to-date, except for a recent driver update released by Intel, but still not available through Dell's update for the graphics card.

Thanks,