My M1 Mac won't detect my LG Ultrawide display

That 34WR50QC-B is a 4K display with the bottom half missing. it supports 8 bits/color. it still needs to meet most of the requirements for a 4K display. it can run at 60 to 100 Hz.

interfaces include:

1x DisplayPort 1.4

2x HDMI 2.0

your M1 Mac supports up to HDMI 2.0

That can run a display up to 4K at 60Hz, but ONLY if your certified cables are PREMIUM or better (ULTRA is better, but not required for this use)

NB> "another" cable is a check against a cable that is Broken. Unless you are also checking specs, it does nothing for a cable that is under the spec required to run the your high resolution and refresh rate. The cables 'found' behind your TV set are generally NOT good enough for high end displays.

if your Mac detects transmit errors, it will cause the display to be reduced in resolution or drop out completely.

Have you tried another cable?

What model LG ultrawide, exactly?

The Mac does not rely on Windows-like side-loaded "Drivers" which are actually packages of resolutions and settings for a specific display. Instead, it goes straight to the immutable source -- it asks the display itself.

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)