You can read the release note of DTK:
https://download.developer.apple.com/Documentation/Universal_App_Quick_Start_Program_Resources/DTK_Release_notes.pdf
Thunderbolt-based displays such as the Apple Thunderbolt Display or LG UltraFine 5K (27MD5KA) are not supported. The LG UltraFine 4K and UltraFine 5K (27MD5KL) support DisplayPort alt-mode and are supported at 4K. The Apple Pro Display XDR is supported at 5K.
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