Monitor does not support HDR playback (Dell U2720Q)

I have a Dell U2720Q external monitor (it is a 4k monitor that supports HDR10) connected to my 2019 16inch Macbook Pro (with AMB Radeon Pro 5500M 8gb graphics card).

I have enabled HDR in the Displays menu:

When I try to play HDR content from Apple TV I get the following error:

Why am I getting this error?

How can I fix this error?

My monitor supports HDR content - so I should be able to play HDR content?

Any help would be really appreciated!