I bought an inexpensive LAN cable tester to give to my daughter.
Above is the sellers pic, the specifications states that it checks data wires 1-8 and the shield / ground connection of STP cables.
On test, it failed to show the shield connection on an STP cable, the LED did not light on either the master or the slave unit.
I tore it apart to see if it was worth getting a replacement.
Above is a pic of the internals of the master.
If you look carefully at the pic, there is a horizontal row of LEDs just below the midline. To the left end, the G or shield LED is missing, there are just two copper pads with no solder.
Peeling the label back on the slave unit reveals the same problem, missing LED and no solder.
The absence of solder suggests that the manufacturing process was not configured to put solder paste or LED in this position, and the master and slave are consistent in having only LEDs 1-8 fitted.
Nevertheless, the RJ45 connectors were STP type connectors, they have the ground / shield connection.
Production runs are probably in tens of thousands quantities, untested!
So, a low quality item that is uncoordinated configuration (STP connector, labels, and no LED), could not have been tested to specification, sold by a seller who doesn't care and bought by unsuspecting buyers… Chinese quality (bit of an oxymoron really).
If you are stuck with one of these and want to fix it, peel the labels off carefully, undo the case screws, add the missing SM LEDs, TEST IT, and reassemble it.
If you are thinking of buying one, skip it… it is more likely to be defective than any other one you buy… but that is no guarantee of success.