Had the same issue here: https://discussions.apple.com/thread/7294305?answerId=29157819022#29157819022&ac_cid=sa123456#29157819
Have copied and pasted my fix below:
Ok I fixed it. I don't know why this is working but at this stage I dont care.
This is exactly what happened:
I read somewhere deep in the bowels of like the third or fourth page of Google that a guy got it working by closing his macbook at the same time as using DisplayPort Wire ("DP")--> Mini DisplayPort ("Mini DP Adapter"
I was using a HDMI cable through the Mini DP Adapter. I thought, ok let me try plugging in the DisplayPort wire at the same time (the adapter has a port for both which can be used simultaneously) then closing the Macbook.
So I plugged in the DisplayPort wire, and nothing happened as I have to change the input to DisplayPort on the Monitor from HDMI. I changed the input to DisplayPort, and it said NO SIGNAL (which was my issue)
Then I closed my bleeping Macbook, and VOILA THE SCREEN BURSTS INTO LIFE FULL 2560x1080 !! So I was happy but also slightly upset that I couldn't use my new monitor with my MacBook open, but hey whatever.
Well, what if I can? So, I try opening the MacBook, AND THEY BOTH WORK PERFECTLY.
PLOT TWIST: All the while, the HDMI cable is still plugged into the Mini DP Adapter and into the Monitor. So I thought, well closing the Macbook must have refreshed something or something else or blah blah, so I can remove the HDMI cable.
I remove the HDMI cable from the Mini DP Adapter AND THE MONITOR DISPLAYS NO SIGNAL!! WHAT IS LIFE!?
SO MY MONITOR IS WORKING WITH A HDMI CABLE AND A DISPLAY PORT CABLE BOTH PLUGGED IN THROUGH A BLEEPING MINI DP ADAPTER WITH THE INPUT SET TO DISPLAYPORT.
Being a man of (reasonable in my own completely unqualified and probably wrong opinion) logic the ONLY THING I CAN THINK OF IS THAT THE MINI DP ADAPTER DRAWS POWER FROM THE MONITOR!? AND THE DISPLAY PORT CABLE WASNT PROPERLY POWERING IT!?
I don't know. At this point, I don't care. Well I do, its still bugging me, but it works. I can't tell you why.