Read this FIRST before posting

All Apple macOS users should FIRST read through =

Dell monitor, Apple FAQ
Dell monitor, Apple FAQ (part 2)

(1) First, check the sticky FAQ Monitor. It has many topics that could address your issue

(2) Where do I find my monitor Service Tag number and 20 digit Serial number?

(3) No cables, power button reset
* Turn the PC off
* Turn the monitor off
* Disconnect every cable (including the power cable) from the monitor
* Press the monitor power button in for 8 seconds (clears any residual power in the circuits)
* Reconnect the power cable to the monitor
* Reconnect only ONE video cable from the monitor to the PC
* Turn the monitor on, then the PC

(4) Run the monitor Reset, Self-Test, BID (Built-in Diagnostics)   
 * Take and post pictures of all of the monitor BID (Built-in Diagnostics) colored full screens showing the issue
Note: If the monitor fails the Self-Test and BID and is still under the purchased 3 year warranty, see this

* Turn the monitor off
* Disconnect every cable (including the power cable) from the monitor
* Press the power button in for 5 seconds (clears any residual power in the circuits). If not a mechanical power button, just wait 5 seconds
* Reconnect ONLY the power cable to the monitor
* Run the BID
- If the BID does not show the issue, the monitor itself is functional. The fault lies with either the cabling, video card/GPU, video card/GPU driver, or operating system
- If the BID shows the issue on the colored and text screens, the monitor is defective

(5) Post your issue in public and all of the following data
* The issue and all of the troubleshooting you have performed
* To what specific PC model is the monitor connected?
* What operating system is running on the PC?
* What specific video card GPU is installed in the PC?
* List all of the video card GPU video out ports
* Then detail how the video card GPU is connected to the monitor like this =
PC GTX 1080 DP out port --> U2718Q DP to mDP cable --> U2718Q mDP in port
PC USB port --> U2718Q USB upstream cable --> U2718Q USB upstream port

Reply to the private message sent to you by the Dell agent. In that private message, provide the following data =
* Monitor Service Tag
* Monitor 20 digit Serial Number
* Pictures of the monitor when running the stand alone BID (Built-in Diagnostics)
* Your full name, shipping address, phone, email address
* If you did not purchase this monitor directly from Dell, they will need a picture of the reseller invoice showing your data and the purchase date. You also have to first fill out the Ownership Transfer form