Dual External monitors not showing signal

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)

When you run an experiment, be sure you have used one of those methods to encourage the Mac to query the display, before you draw any conclusions.

For reference, here is the manufacturer's description of that dock:

UGREEN – Revodok Max 213 Thunderbolt 13-in-1 Docking Station

It appears to be a Thunderbolt 4 dock, with three places to plug in displays:

• A DisplayPort v1.4 port ("8K@30Hz, 4K@120Hz, 2K@144Hz")
• Two downstream Thunderbolt ports ("8K@30Hz, 4K@120Hz, 2K@144Hz")

They claim that on Macs with something other than plain M1/M2/M3 chips, you can plug in two displays at the same time, either by

• Plugging displays into the downstream Thunderbolt ports, or
• Plugging one display into the downstream Thunderbolt port and one into the DisplayPort

How are the displays connected to the Mac?

What hub or dock are you using?

Does the non-function la display show anything at all when both are connected?

Do the displays appear in System Settings ➜ Displays?