We are having issues getting the Dell P2715Q monitor to run at 4K and 60HZ.

We have a 5K Retina iMac and the display is connected through Dell's miniDP to Displayport cable that came with the display. All is great (4K and 60Hz), but the minute we plug in our second display, a 1080p television, through a belkin thunderbolt to HDMI the Dell turns very ugly. The only way to get 4K back is to switch on MST, but then the display switches to 30hz..

How can we get the display to run at 60Hz and 4K while the TV is also connected? Surely this must be possible?