The display must be present at certain times, and Must respond with its capabilities.
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)
also, cables that are too long or not quite good enough can cause transmission errors, which later macOS checks for and reduces the resolution until the errors stop.
ThunderBolt displays should have Apple cables no longer than 0.8 meters, Brand X cables no longer than 0.5 meters.
DisplayPort cables and USB-C cables (for displays) should be limited to one meter.
... and YES, I do mean total length, unless there is re-driver circuitry in you KVM or cables.
