Re: #2 U2518D, Smart HDR greyed out, not available, MacBook
ovodesign82 has not replied to that thread since 4/19/18.
Dell can only assist you with the monitor if you connect it to a Windows PC. You can see here under, "Operating system", that the Dell monitor was never tested/validated by Dell with the Apple proprietary hardware and operating system. If you are going to connect it to the non-validated Apple product, you would be better served to post the issue on the Apple Forum.