Monitor doesn't work

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Laptop: Lenovo ThinkPad T420s

OS: Fedora 25

Monitor: Dell 27" Flat Panel P2715Q

Connection: DP (laptop) to mDP (monitor)

xrandr output: see below

I'm a consistent i3 user (no DE), but I've even tried to get this monitor working with gnome. Nothing. The monitor (and Fedora, judging by the xrandr output) both seem to be "talking" to each other (for lack of a more educated comment). But there is no actual picture showing up on the 2nd monitor. The monitor says "there is no signal coming from your computer", but if the cable wasn't attached it would have a different error message.

Any thoughts on how to get this working? In gnome, I tried every variation of every resolution (multiple Hz settings as well), still nothing is working. Is there a driver I need to download or something?

Thank you in advance.

Screen 0: minimum 8 x 8, current 1600 x 900, maximum 32767 x 32767
LVDS1 connected primary 1600x900+0+0 (normal left inverted right x axis y axis) 310mm x 170mm
   1600x900      60.00*+
   1368x768      60.00  
   1280x720      60.00  
   1024x768      60.00  
   1024x576      60.00  
   960x540       60.00  
   800x600       60.32    56.25  
   864x486       60.00  
   800x450       60.00  
   640x480       59.94  
   720x405       60.00  
   640x360       60.00  
VGA1 disconnected (normal left inverted right x axis y axis)
VIRTUAL1 disconnected (normal left inverted right x axis y axis)
LVDS-1-2 disconnected (normal left inverted right x axis y axis)
VGA-1-2 disconnected (normal left inverted right x axis y axis)
DP-1-1 connected (normal left inverted right x axis y axis)
   3840x2160     29.98  
   2560x1440     59.95  
   1920x1200     59.88  
   1920x1080     60.00    50.00    59.94    24.00    23.98  
   1920x1080i    60.00    50.00    59.94  
   1600x1200     60.00  
   1680x1050     59.95  
   1280x1024     75.02    60.02  
   1280x800      59.81  
   1152x864      75.00  
   1280x720      60.00    50.00    59.94  
   1024x768      75.03    60.00  
   800x600       75.00    60.32  
   720x576       50.00  
   720x576i      50.00  
   720x480       60.00    59.94  
   720x480i      60.00    59.94  
   640x480       75.00    60.00    59.94  
   720x400       70.08  
DP-1-2 disconnected (normal left inverted right x axis y axis)
DP-1-3 disconnected (normal left inverted right x axis y axis)
  1024x768 (0x124) 65.000MHz -HSync -VSync
        h: width  1024 start 1048 end 1184 total 1344 skew    0 clock  48.36KHz
        v: height  768 start  771 end  777 total  806           clock  60.00Hz
  800x600 (0x127) 40.000MHz +HSync +VSync
        h: width   800 start  840 end  968 total 1056 skew    0 clock  37.88KHz
        v: height  600 start  601 end  605 total  628           clock  60.32Hz
  640x480 (0x12b) 25.175MHz -HSync -VSync
        h: width   640 start  656 end  752 total  800 skew    0 clock  31.47KHz
        v: height  480 start  490 end  492 total  525           clock  59.94Hz