From their manual at
It looks like the modem is doing the character conversifroze tween Baudot and ASCII - which is good otherwise you might have been stuck.
It says that you must set the serial connection to it as 300 baud, 8N1 (8 bit, no parity, 1 stop bit) .... but maybe that is not right because you said you thought 2400 baud works.
In any case, it is not clear whether or not it is using hardware flow control because the wording is imprecise.
The manual certainly says no software flow control (I.e. xon/xoff or ctl-s/ctl-q).
Looping together the 2 pins in the DB9 plug might work anyway if it needs a signal and you can't generate one. (pin numbers are standard and are listed at the end of the manual).
FYI I started my computing career on TTY (Teletype - not TDD) writing applications and device drivers to handle slow speed serial comms and remember having to generate test messages on paper tapes. Some of the systems had "fractional speed" trans-atlantic links where the 50 baud connection was split between 3 terminals at the far end ...
Biggest system we did was to support 192 asynchronous connections from a PDP-11 84 - with the slow speed links going out to teletypes on oil rigs.