Latitude 9520 poor resolution on second screen

Hi, can anybody please help me?  I've just taken receipt of a new Dell Latitude 9520, top of the range convertible laptop from Dell and its great except, when I connect to my second Dell screen, the resolution on this screen is poor. My new 9520 is replacing an old Microsoft surface Pro which was previously connected to this second screen and the resolution was fine, so its not an issue with the screen. I have also swapped cables, and its not a cable problem. I'm connecting the screen through a hub connector via HDMI cable, and Ive tried connecting the HDMI cable directly to the 9520 (instead of through the hub) and this changes nothing.

Finally, I've played with all of the display settings on the 9520 , but cannot improve the resolution.

Is it normal that I have a poorer 2nd screen resolution with this brand new Laptop when compared to my old Surface Pro or does anyone have any ideas, which I would appreciate enormously? 

thanks a lot