You purchased an Inspiron 3910 desktop with only the onboard integrated Intel UHD 770 graphics. This means that you only have access to the onboard integrated HDMI 1.4 out port, DP 1.4 out port and the Intel UHD graphics control panel and driver. To have access to the AW2721D Nvidia G-Sync Ultimate higher Hz above 60, you should have purchased the additional Nvidia GeForce GTX 1660 Super video card. See page 19.  The AW2721D should still function at 2560x1440 60Hz using the onboard integrated DP out port. While connected via ONLY the HDMI to HDMI cable, open the AW2721D Menu. Go to Others and run Factory Reset. Exit the Menu and then turn the AW2721D and Inspiron 3910 off. Disconnect the HDMI to HDMI cable from both. Connect the DP to DP cable to both. Power both on.
Just out of the box with brand new Inspiron 3910 using onboard Intel UHD 770 graphics. Had to set up using HDMI because no signal detected via DP (DisplayPort). All using cables that came with the unit. Updated Windows and searched for all latest drivers. Reset multiple times. No help. The entire reason I bought this AW2721D is to get 144Hz refresh rate which needs DP.   
Inspiron 3910, AW2721D won't detect DP