If you are 90% sure you don't have a null modem cable, unfortunately the Keyspan will give you nothing more than you already have. You still need a null modem cable plugged into the 9 pin male plug of the USB/serial converter whether it's a Keyspan or a generic unit.

The RS232 (and other similar) protocols allowed you to connect a terminal (ie your computer) to a modem via a simple straight wired cable. However you are not connecting to a modem, you are connecting to another computer, what is in RS232 terms another terminal. So a crossover cable is needed called a null modem cable because there are no modems ie terminal-modem-modem-terminal. Its the modem-modem bit that does the crossover - think phone handsets where crossover is because earpiece and mouthpiece are 'cross connected' either electrically or acoustically. The null modem cable simply mimics this.

ie
terminal-straight-modem-cross-modem-straight-terminal
or
terminal-cross (null modem)-terminal

Adding a USB/serial converter changes nothing. The plug on the USB converter is the same as would be on a terminal device and does not magically change it into a modem type device. So the cabling requirements would be the same whether or not the USB/serial adaptor was used or not and a null modem crossover cable is still required.

The unfortunate part is that a standard female-straight-female cable is often visually indistinguishable from a null female-crossover-female cable and many box of bits contain the straight cable from when we connected to external modems. Many people not use to straight and crossover cables also confuse female/female cables as meaning null modem when it's the non visible internal wiring that determines the type.

Meridian owner since 1992
Prime & PSU, Focal Elear headphones, Roon (ROCK 8Gi5 NUC), Explorer 1 & 2, F80. 200/203, various Sonos (via Roon).