The Best My External LG 5K monitor can do is only 4k - Why?

from the owner's manual, here:

https://www.lg.com/us/support/product/lg-34WK95U-W.AUS

Troubleshooting:

4K/5K resolution is unavailable.

Did you set the DisplayPort Version to 1.4? (In case of 5K)

Use on-screen display settings:

• Go to Settings > General > DisplayPort Version to set the version to 1.4.

The Mac uses a system that reminds me of “Plug and play” to determine what display is connected, and what its capabilities are.

To get a Mac display to become active, you need the Mac to query the display, and the display to answer with its name and capabilities. Otherwise, the display will not be shown as present, and no data will be sent to the display. "No signal detected" is generated by the DISPLAY, not by the Mac.

This query is only sent at certain times:

• at startup

• at wake from sleep — so momentarily sleeping and waking your Mac may work

• at insertion of the Mac-end of the display-cable, provided everything on that cable is ready-to-go

• hold the Option key while you click on the (Detect Display) button that will appear in Displays preferences (from another display)

These are the ONLY times when the screen resolution will be re-evaluated.

When users have wanted to investigate what recommended EDID resolutions were being provided by displays, SwitchResX was the software they used to read out the information provided by the display.

Maybe Apple support can help. The first responders have a script they follow. Once they have checked off all the boxes, they MAY try to tell you all is well. Instead, ask for a specialist to contact you, and do not give up until you get one of:

• This is a known problem, we are actively working on fixing -OR-

• I will file Bug Report on your behalf -OR-

• take your Mac to this Apple Store, and WE will instruct them to fix it for you.

Official Apple Support

Your ThunderBolt cable needs to be rated for 40 G bits/sec and shorter than 0.8 meters long to support the highest speeds.

SwitchResX is a shareware investigative tool. It likely does not meet the standards of the Mac App Store. But it does appear to have a Developer Certificate and can be installed easily;

https://www.madrau.com/srx_whatitis/whatitis.html

'beamer' is German name for a projector.

what shows in (current resolutions)?

(Those are the standard ones built into the display.) There should be a 5120 resolution showing there, else your display has a defect.

Detect Displays says go ask all possible displays to reveal their names and resolutions. When a display does not display at all, that can make it active. or it can make it re-state its capabilities after internal changes.

what you are describring sounds like (identify displays) or (gather windows) which used to put a number up on each display, (or a red box in a different version). Then it would show settings of each display, transcribed onto the main display's window. I don't know whether that is still supported.

Thanks for reporting back, that is good news for you, as a result of your continued pressure against the machinery!

Editorial: is seems than additional complexity of both the controllers and the software that runs these high-end displays means they get confused FAR more often than older, simpler displays. The newest Apple display is a prime example where comparatively often, the display develops a problem and needs to be reset to solve the problem.

If using SwitchResX makes you feel queasy, My next suggestion would be to contact the display-maker for additional assistance.