I am having the same issue with a Mac Pro late 2013.
I have daisy chained two U2518D, both with DisplayPort 1.2 and MST enabled. I have also tried DP 1.2/MST Enable in the first U2518D and DP 1.2/MST Disable in the second U2518D. But I only get mirroring the screens, not extending them.
My current setup is the following: Mac Pro mDP --> DP in #1 U2518D (DP 1.2/MST Enable) #1 U2518D DP out --> mDP in #2 U2518D (DP 1.2/MST Disable)
But the screens are mirrored instead of being extended. Please, could someone tell me what I am doing wrong?
For my internal notes, click my name and private message me both of the U2518D service tag numbers.
For Dell to troubleshoot, you need to test the U2518D's on a dedicated Windows operating system PC to see if the daisy chain issue is present.
We can see on the U2518D drivers page and also in the U2518D User's Guide that there is no mention of the proprietary Apple operating system or hardware. You should read the U2518D threads on the Apple Forum and see what other Apple users have done as far as Apple PC troubleshooting and possible Apple proprietary cabling to purchase. If need be, sign in there and start a new thread.