Only after ordering three serial-to-TTL converter boards with level translators for both RX/TX and RTS/CTS (labeled “WaveShare” and featuring the Sipex SP3232), I figured those are made to connect a microcontroller as a peripheral. The seller must have had reasons to provide no photos of the DB9 port, from which it would have been too obvious that it only has a female connector.
The issue was easy to solve, though. Remove the female DB9 connector, replace it with its male counterpart (attention: the pinout is mirrored, so the connector must be mounted on the opposite side of the PCB), swap input and output connections of the level converter chip (don’t forget to cut the original traces on the PCB), and voila. Given the SP3232 only requires few external components and the board closely follows the reference layout, the job was finished within a matter of minutes.