Thanks @stevelup ; when the Switch is connected via the Airport Express, it is showing CONNECTED (100 FDX)
, whereas all of the other switches just say CONNECTED
, which I assume means better/faster.
Most googling shows this 100 FDX issue concerned with access points rather than switches; the Unifi FAQ has "The UAP is physically wired to the network at 100 Mbps in full-duplex mode. This will appear when a UAP is connected but not at the ideal connection rate. FDX stands for Full Duplex. It may appear as 10/100/1000, HDX or FDX.
If I use a new cable between the splitter widget and the switch, the port light does not come on at all. So it looks like the Airport Express is somehow "boosting" the signals within the ethernet cable, despite the proximity of the Switch.