I have that same monitor. the problem for me was the mini displayport on my r9295x2.
Xfx had to send me a whole new card.
The other thing you can try are the best DP cables you can buy. Gold plated, best reviews, etc. You might have to try 2 or 3 different brands of cable.
If that dosent work then you'll have to RMA the monitor. If that still dosent work than its the DP ports on your 980.
I went through this with the PB287Q. I RMA the monitor, waited 4 weeks to get it back and.......same problem. RMAD the 295x2 and turns out all 4 mini displayport were dead on the card.
Total of 6 weeks of frustration to find the source of the no displayport error.
so it's iether the cable, the monitor, or the card.
Good luck......