Question about Dell U2715h

I have the same configuration and with the Thunderbolt2 connection it works without problems; the only important thing is that you attach the display with the MBPr open and only then close the lid.

To all those who have trouble connecting this monitor, make sure you don't mix up the display out ports and the IN ports :)

The problem with this monitor model is that you have to be very careful which port you choose. I also had a lot of trouble until I took a closer look at the ports. Some are out ports. You have to connect the cable to one of the In ports. Unfortunately, you can't see that unless you use a torch and turn your head.

Try it like this: Switch on the screen or put it in stand-by, switch on the MBPr, connect the MBPr to power, connect the display port cable to the MBPr; now you can run the MBPr with the external screen.
If this does not work, then the problem can be caused by many things: HW MBPr, HW Screen, OSX, Settings MBPr or Settings Screen. Unfortunately, I can't help you any further.