I recently purchased a Dell Ultrasharp 27" monitor U2719D to use as a second monitor to my iMac 27" 2017. The Dell U2719D came with two cables= DP to DP cable, USB 3.0 upstream cable. On the back of my iMac I have four USB-3 slots and two Thunderbolt 3 (USB Type-C) slots. I am wondering how to connect the U2719D to the iMac? I have tried using the USB 3 cable directly from U2719D to iMac, but the iMac doesn't show a second monitor under preferences. I bought a USB 3 to USB-C adapter and put this on the USB-3 cable and then plugged it into the Thunderbolt 3 slot on the iMac but still the second monitor doesn't show under iMac preferences. I am wondering what I need to do to connect them successfully?

Thanks

CJ