Thanks for getting back. We were using the wiring table shown on that exact page.
TxD(A)/RxD(A) Transmitted/Received Data (A) 4
TxD(

/RxD(

Transmitted/Received Data (

5
GND Ground 6
Since TX & RX share the same pair, the fact that we got the TX part work correctly indicates the wiring is fine. We still have no idea why RX does not work.