SWITCHRESX MOJAVE UPDATE
SWITCHRESX MOJAVE DRIVER
SWITCHRESX MOJAVE PRO

Not sure if this is a solution but in working various configurations to see what was most stable what appears to be working for me now especially after waking my computer from sleep isДЎ) having SwitchResX installed ( I went ahead and purchased copy) and giving me easy access to all available resolutions (obviously SwitchResX is simply mirroring what is available and not 'creating' new options - but it is far more resilient in this respect than the Display Pref Pane) - I have it in basic mode and w/o any custom resolutions setup Дў) turned off Automatic Graphics Switching in the Energy Saver Preference Pane.

SWITCHRESX MOJAVE UPDATE

One quick update - yet to test the new cable (but of course the OP had gone this route too w/o success apparently - TBD).

SWITCHRESX MOJAVE DRIVER

Having nothing else in line in my setup (i.e.as simple as it gets), it seems that the Thunderbolt or USB C driver seems to be having difficulty being consistent with deciding whether or not the output port has full bandwidth or not available - this ultimately is indeed an Apple issue that I wish to heck they'd get on top of (already reported but many as well be in the ether). variable results on wake up from sleep when you'd see 30 Hz available when previously you had 60 Hz - so all in line with what I am experiencing. Stephane Madrau the developer of SwitchResX commented that as with other 4K displays he has numerous reports of what appear to be unreliable responses from the Apple USBC/TB3 ports i.e. And just to add to the picture in as I write this response SwitchResX is now showing me 2 separate options to work at 5120x1440 both at 60Hz !!, which I can select and work, and the 30 Hz option is also still available - this and a load of other resolutions 'magically' appeared (possibly after a wake-up - def not a reboot)! Thx - yes had thought of that and wondered if the supplied USB C cable was really up to snuff - plan to use a higher end cable arriving tomorrow to test that - also need to recheck specs of each port as I am told that not all are equal in respect bandwidth capability (true? - heard that the slots on withe rside of 13' MacBooks differed but have not checked re my 15').

SwitchResX in basic mode with system CSRUTIL enabled again appears to be holding onto it and I can still currently access it via that tool (which I will purchase if it remains useful but still a bit skeptical).ДЄgain hoping that others have insight into solutions as more of these displays get used. Perhaps even more annoying though is that w/o SwitchResX the login screen in Mojave would be set to the perfect resolution (so clearly the system is seeing and providing an option here) yet when you get fully logged to the desktop the system reverts to presumably a 'safe resolution' or blessed resolution set and loses the 5120x1440 option (not visible with option modifier in the pref pane).

If you have constructive advice feedback on solutions pertinent to this discussion I am all ears - providing motherhood statements as to hardware configs does nothing to move this conversation forward.ДЄt this time SwitchResX is one option which appears to at least provide more consistent access to 5120x1440 but lately apparently only at 30Hz (despite it too seeing the 60Hz option previously!) - I have done this with both CSRUTIL enabled and disabled and currently the disabled state with only their Dasmon on (no helper tools) is working best.

SWITCHRESX MOJAVE PRO

The point here is that 2017-2018 MacBookPro HW is more than capable of handling these resolutions and even showed the option of 5120x440/60Hz on initial hook-up in the Apple Displays Pref Pane (5120x1440 ow resolution)! Further the packaging and advertising from LG specifically highlights the MacBook Pro working with this display (regardless of what you think of any guarantee from them of course).

My input above is building on and sharing my experience with this Display following the OP's post and clearly looking for preferably constructive solutions from a user community as Apple has essentially provided zero advice/assistance on this. Perfectly aware I am on a user forum and of issues with graphics hardware.

Switchresx mojave