While holding down the SHIFT key, right click on the DDM system tray icon and select "Save diagnostic report" from the popup menu. Then post the content of the file you saved here. The report will show what is wrong.

Looks like it doesn't like my graphics card. AMD Radeon RX 5700 XT.   I swapped my old card back in place, and DDM worked fine.  In both logs files, it identifies the monitor, but as you can see below, it states the display is unsupported.  I'm not sure why it would care about the card - which is using the identical drivers because both are AMD.  Both are using DP1.2 to communicate with the monitor as well.

Diagnostic Report, generated 7/10/2019 (10.0.18362)
Copyright (c) 1995-2019, EnTech Taiwan.
Version = 1.4.0.1942
OS = 10.0.18362.2 x64
Mechanism = 3
Account = Standard
Command line = "C:\Program Files (x86)\Dell\Dell Display Manager\ddm.exe" ExpandConstant
////////////////////////////////////////
DriverError = 0x00000000
LastCheck = 0x0000AA88
EnumErrorCount = 0x00000000
HintShown = 0x00000001
////////////////////////////////////////
19:32:38.359...Log opened
19:32:38.406...LanguageID = 00000409 00000409 00000409
19:32:38.421...Build = 1.4.0.1942
19:32:38.421...Windows handles = 0002012A 000100C4 0001012E
19:32:38.421...OS monitor sleep timeout value = 600 ms.
19:32:38.421...Applied DPI = 96-96
19:32:38.421...VM = 0
19:32:38.684...Starting enumeration
19:32:38.934...1. DELA0F3.97F8P85A0N8L on AMD Radeon RX 5700 XT
19:32:38.934......Monitor assignment = 1
19:32:38.934......Adapter name = \\.\DISPLAY1
19:32:38.934......Monitor name = Dell U3818DW(DisplayPort)
19:32:38.934......Vendor ID = 1002
19:32:38.934......GPU driver = aticfx64 8.17.10.1669
19:32:38.934......Flags = 00080005.00000003
19:32:38.934......Display tag = CC4CB5F2
19:32:38.934......Monitor number = 1
19:32:38.934......Monitor handle = 00010001
19:32:38.934......Raw EDID = 00FFFFFFFFFFFF0010ACF3A04C384E30131C0104B55825783EEE95A3544C99260F5054A54B00714F81008180A940D1C00101010101014C9A00A0F0402E6030203A00706F3100001A000000FF003937463850383541304E384C0A000000FC0044454C4C20553338313844570A000000FD001855197328000A20202020202001CC
19:32:39.295......Error: communicating via DDC/CI (0x0000001F) = A device attached to the system is not functioning
19:32:40.255......Error: failed to obtain output mask from display driver
19:32:43.099......Error: communicating via DDC/CI (0x0000001F) = A device attached to the system is not functioning
19:32:44.080......Error: failed to obtain output mask from display driver
19:32:49.442......Error: communicating via DDC/CI (0x0000001F) = A device attached to the system is not functioning
19:32:50.369......Error: failed to obtain output mask from display driver
19:32:50.369......Discarding unsupported ;DEV_731F&SUBSYS_0B361002&REV_C1
19:33:15.932.....DeviceKey = \Registry\Machine\System\CurrentControlSet\Control\Video\{F2630291-A3EE-11E9-8006-86194EE65FA4}\0000
19:33:15.932.....a.MonitorName = \\.\DISPLAY1\Monitor0
19:33:15.932.......MonitorFlags = 00000003
19:33:15.932.......MonitorString = Dell U3818DW(DisplayPort)
19:33:15.932.......MonitorID = MONITOR\DELA0F3\{4d36e96e-e325-11ce-bfc1-08002be10318}\0003
19:33:15.932.......MonitorKey = \Registry\Machine\System\CurrentControlSet\Control\Class\{4d36e96e-e325-11ce-bfc1-08002be10318}\0003
19:33:15.932.......Raw EDID = 00FFFFFFFFFFFF0010ACF3A04C384E30131C0104B55825783EEE95A3544C99260F5054A54B00714F81008180A940D1C00101010101014C9A00A0F0402E6030203A00706F3100001A000000FF003937463850383541304E384C0A000000FC0044454C4C20553338313844570A000000FD001855197328000A20202020202001CC
19:33:15.932...2.DeviceName = \\.\DISPLAY2
19:33:15.932.....DeviceFlags = 00080000
19:33:15.932.....DeviceString = AMD Radeon RX 5700 XT
19:33:15.932.....DeviceID = PCI\VEN_1002&DEV_731F&SUBSYS_0B361002&REV_C1
19:33:15.932.....DeviceKey = \Registry\Machine\System\CurrentControlSet\Control\Video\{F2630291-A3EE-11E9-8006-86194EE65FA4}\0001
19:33:15.932...3.DeviceName = \\.\DISPLAY3
19:33:15.932.....DeviceFlags = 00080000
19:33:15.932.....DeviceString = AMD Radeon RX 5700 XT
19:33:15.932.....DeviceID = PCI\VEN_1002&DEV_731F&SUBSYS_0B361002&REV_C1
19:33:15.932.....DeviceKey = \Registry\Machine\System\CurrentControlSet\Control\Video\{F2630291-A3EE-11E9-8006-86194EE65FA4}\0002
19:33:15.932...4.DeviceName = \\.\DISPLAY4
19:33:15.932.....DeviceFlags = 00080000
19:33:15.932.....DeviceString = AMD Radeon RX 5700 XT
19:33:15.932.....DeviceID = PCI\VEN_1002&DEV_731F&SUBSYS_0B361002&REV_C1
19:33:15.932.....DeviceKey = \Registry\Machine\System\CurrentControlSet\Control\Video\{F2630291-A3EE-11E9-8006-86194EE65FA4}\0003
19:33:15.932...5.DeviceName = \\.\DISPLAY5
19:33:15.932.....DeviceFlags = 00080000
19:33:15.932.....DeviceString = AMD Radeon RX 5700 XT
19:33:15.932.....DeviceID = PCI\VEN_1002&DEV_731F&SUBSYS_0B361002&REV_C1
19:33:15.932.....DeviceKey = \Registry\Machine\System\CurrentControlSet\Control\Video\{F2630291-A3EE-11E9-8006-86194EE65FA4}\0004
19:33:15.932...6.DeviceName = \\.\DISPLAY6
19:33:15.932.....DeviceFlags = 00080000
19:33:15.932.....DeviceString = AMD Radeon RX 5700 XT
19:33:15.932.....DeviceID = PCI\VEN_1002&DEV_731F&SUBSYS_0B361002&REV_C1
19:33:15.932.....DeviceKey = \Registry\Machine\System\CurrentControlSet\Control\Video\{F2630291-A3EE-11E9-8006-86194EE65FA4}\0005
19:33:15.932...7.DeviceName = \\.\DISPLAY7
19:33:15.932.....DeviceFlags = 00080000
19:33:15.932.....DeviceString = AMD Radeon RX 5700 XT
19:33:15.932.....DeviceID = PCI\VEN_1002&DEV_731F&SUBSYS_0B361002&REV_C1
19:33:15.932.....DeviceKey = \Registry\Machine\System\CurrentControlSet\Control\Video\{F2630291-A3EE-11E9-8006-86194EE65FA4}\0006
19:33:15.932...8.DeviceName = \\.\DISPLAY8
19:33:15.932.....DeviceFlags = 00080000
19:33:15.932.....DeviceString = AMD Radeon RX 5700 XT
19:33:15.932.....DeviceID = PCI\VEN_1002&DEV_731F&SUBSYS_0B361002&REV_C1
19:33:15.932.....DeviceKey = \Registry\Machine\System\CurrentControlSet\Control\Video\{F2630291-A3EE-11E9-8006-86194EE65FA4}\0007
19:33:15.932...Total number of monitors enumerated = 1
19:33:15.932...\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\

I'm afraid the problem is with the Radeon RX 5700 XT driver - they seem to have broken their DDC/CI support  (required for communication with the monitor) with the driver build you are using.

19:32:39.295......Error: communicating via DDC/CI (0x0000001F) = A device attached to the system is not functioning
19:32:40.255......Error: failed to obtain output mask from display driver

The first error message is from Windows; the second from the AMD ADL.

This is the same exact driver used for the Radeon R9 390 that works. Same driver and same version.

You'd have to ask AMD about this result - the two errors are from Windows and the AMD ADL, not DDM or the monitor.

The only other possibility is if you were going thru some sort of external KVM, input converter or hub. But if you are connected directly from the 5700 to the U3818DW, then the only explanation and solution would have to come from AMD.

Have you tried an HDMI connection? If that were to work, where DP didn't, you'll have helped AMD localize the problem.

I have this same issue trying to use software to control my screen brightness through DDC/CI.  I had a 2070 beforehand and had no issues with it.  After I switched to a 5700 XT now it doesn't work with my Acer XFA240 monitor, it says no compatible monitor detected (using Displayport). Weird thing is it works with my Asus monitor fine that has HDMI. So it seems like a driver issue using 19.9.2 and the 5700 XT. I have submitted this bug report to AMD directly through the Radeon Settings, hopefully they fix it in a future driver update.