HI,
I just got my 49" DELL today and faced the same problem. In SwitchResX, there was a 5120x1440 mode, but it was upscaled by using 3840x1080 and looks terrible, even if you put the monotor sharpening to 100%. After an hour of trials, 5120x1440 still does not work, but I managed to get 2x 2560x1440 screens by using the PBP mode. What I did in details:
1) re-addressing the USB1 and USB2 outputs on the monitor - one was addressed to USB-C, which may reduce its bandwith
2) attaching two cables USB-C to DP and USB-C to USB-C
3) switching on PBP mode. At the start, this results in two screens both with 3840x1080
4) Start SwitchResX and define a 2560x1440 mode with 60Hz
5) Restart the Mac
6) Start SwitchResX and have a look at the table. The 5120x1440 upscaled mode should be gone, but there should be a new mode 2560x1440. And instead of the one in the default table, this one is in bold, which means it is supported in native mode. If it does not appear in the first place, change the current resolution and try another reboot.
Now you get a full screen 2x2560x1440 resolution, which consists of two virtual displays. This leads to some display issues, which I solved in the following way:
1) You may have now two menu bars on the top. I switched of the "monitors use different spaces" function in "Mission Control". After a log-out and log-in, there is just one menu bar left - which starts at the middle of the screen - and thankfully reduces mouse moves a bit. Acceptable for me. More important: windows can then share more than one desktop and allow to be moved seamlessly between the virtual displays. Otherwise you end with a window cut at the middle of the screen.
2) I also had to create a mirrored background image for the right screen to hide the position where the two deskltop images come together. As I use some kind of color gradient picture, this was done easily.
This is a work-around, of course. But it is much better than using a way-too-big resolution with only 3840x1080 (which would be also a waste of money, as monitors with that native resolution are way cheaper).
The monitor works fine with Windows, which lets me hope that Apple will address this issue in the future. Besides, the same issue was reported with other 49" monitors by LG...