The U2415 has two HDMI in ports, #2 HDMI MHL 1 and #3 HDMI MHL 2.
* 3650 desktop HDMI out port --> HDMI to HDMI cable --> U2415 HDMI in port
* Turn both on
* Press the U2415 Menu button
* Go to Input Source
* Choose the HDMI port you used
If you got it working via HDMI to HDMI at 1920x1080 60Hz, you could leave it there. Or install the Dell 1MPR3 AMD R9 360 (DVI-I/DP/HDMI) video card. Then you could get 1920x1200 60Hz using a DP to DP cable.
I have 1920x1080 60Hz. Is there a performance advantage to a new card?
BTW, the people on the chat support system had no clue how to fix my problem.