U2718Q, vertical lines

Since you are seeing those vertical lines on the U2718Q no video cable Built-in Diagnostics, you have an internal hardware component failure. Dell does not repair monitors. All Dell would do is replace it while still inside the originally purchased warranty period.

Click the DELL-Cares username. On the right, click, "Send this user a private message". Share the private U2718Q Service Tag with them so they can check the warranty status.