I can confirm that if the IAP is setup to work as a 2-port device, then the LED are only for the Screw terminals - not for the DB9.
In 'pass-through' mode, the data is 'repeated' out both ports during TX and 'merged' by software during RX, so the TX LED will work by accident (because of the unused TX echoed out the screw terminals), but there will be no RX LED as that is tied to the idle terminal block.