@ittakes2bananas  Since the Mac Mini uses the HD 630, it likely has the same issue that was described in this thread, where I provided a theory as to why you can't run 5120x2160 on that display even though the system is capable of 5120x2880.  Very basically, there are different ways of running high resolutions through Thunderbolt, and if the system only supports Method A and the display only supports Method B, you're stuck.

I got the same problem. However, the boot camp win10 works perfectly fine with 4021qw. You will have 50% chance to enter into Mac OS. But in low resolution. Considering to send 4021 back.

sorry, I need to correct myself. You report is right. The boot camp win10 can go up to 5120x2160 but only at 30hz. Try to search on internet, all command line thing. But nothing worked. So, I tried to use typec to hdmi adapter and hdmi connector, then use 4021qw’s pip mode, split it into 2 equal screen. Everything is fine, just 2 screen, Mac and win10 both ok at 60hz. Anyway, I believe I will split it because it is too big. This is the temporary solution, I have heard that dell is not active in Mac compatibility. So might be the only possible way to use it now. Wish dell or apple could do something to correct it. This is way too stupid to leave it there.

@Hali322  Thanks for following up.  Based on your post I wasn't sure how to account for what was going on, since if you had been able to run 5120x2160 at 60 Hz from that system, it would have completely ruled out my theory.  But 30 Hz makes sense.  And being able to run the display as two separate tiles would also be expected, since the system is capable of providing dual GPU interfaces that together can run that display at 60 Hz -- but apparently the display will only accept being driven that way in PBP mode.  Of course running PBP with both tiles connected to the same system isn't quite the same experience as having a true single display, since setting something to full screen will only make it full screen on a single tile, and dialog boxes that are supposed to be centered on the display will only be centered on one tile.  But it's certainly better than nothing, I guess.

In terms of future improvements, I personally would not be optimistic, and in fairness this isn't quite like other Dell/Mac interoperability issue.  Those issues are cases where everything indicates that something should work one way, and yet it doesn't.  In this case, if my theory is correct, this is a case where the Mac only supports one method of running 5120x2160 (dual DP 1.2 interfaces aggregated), and the display only supports a different way (single DP 1.4 interface).  So this is just a situation where the incompatibility is expected once the information is known, not a case as has occurred elsewhere in Dell/Apple setups where there's no clear explanation why something isn't working as it should.  But in terms of fixes, systems that only support DP 1.2 can't be updated to support DP 1.4 since that's a hardware limitation.  As for the display, I don't know if it would be possible to add support for aggregated DP 1.2 interfaces with a firmware update.  If so, I agree it would be a welcome enhancement, and in fact that would even benefit some PC setups.  But the fact that Dell used aggregated DP interfaces for their original 5K display and more recently on their 8K display (which uses dual DP 1.4 interfaces for 8K 60 Hz) but did not do that on this display makes me wonder if there's some reason that it wouldn't be feasible.

@jphughan  Thanks so much for the thorough follow-ups and information. Fortunately for me, most of my day is spent on my work 2019 Macbook 16" that runs the display perfectly using USB-C->USB-C at scaled 3840x1620 60hz. So I will just live for the 5120x2160 30hz on the '18 Mac Mini for now — I use it primarily for Logic Pro, which has some settings for menu/text enlargement, so it's not much of a hindrance other than the refresh rate not being ideal.

Interestingly, I saw the 11.3 macOS update that released earlier in the week had reports of fixing some ultra-wide issues on the Mac Mini so I was hopeful this may provide some improvements. But it only took a minor step back...

I had been able to get 5120x2160 30hz via HDMI -> HDMI which I had been using the past few weeks. However, this update capped the resolution at 3840x2160 30hz using HDMI. I've had to switch to a USB-C -> HDMI cable to get back to where I was  . Seems the OS update only helped with M1 macs.

One other thing I wanted to note...if I connect the monitor either by USB-C->USB-C or USB-C-> DisplayPort, while the Mac Mini is running then "No signal is detected". However, if I turn the Mini off and restart with the monitor connected, it will boot up and get to the login screen (looks like it's running 2560x1080) — but after entering my password the loading bar will get very near the end and then the monitor "glitches" into a bunch of weird shapes/colors and as soon as the loading bar completes the monitor will go black saying "no input detected". Strange!

@ittakes2bananas Happy to help.  Wish I had better news for you about the issue though!  Yes, my understanding is that the update for ultra-wide resolutions pertains only to M1-based systems.  In terms of having fewer resolution choices, if you haven't already, you might want to check out a utility called SwitchResX, which gives you quite a few more options when it comes to resolution and scaling.  In terms of the glitchy behavior on startup/login, that sounds like a firmware issue.  Given that both the display and the system are relatively new, it's anyone's guess as to which device is "at fault" here, but it's also possible that a future firmware update will resolve this.  I'd keep an eye on the Dell Support site for firmware updates to that display -- although you may need a Windows system to actually install it if one arrives....

Have you tried SwitchResX ?  

Worth googling...

Good point.

Considering one of these for my Big Sur Hackintosh / 2018 MacBook and trying to figure if it's doable...

@Electric_Haggis  Great app, but if the system and display can’f negotiate a link that supports enough bandwidth to run the desired display setup, then a software application that allows you to force various resolutions isn’t going to solve that.

Did you ever find a solution to this problem? I'm in the same place.