It sounds like the M app is looking at comm ports 1 to 4 and you USB/serial converter has chosen a port above this range - which is normally the case. Hopefully it will simply be a matter of in the Windows device manager of simply changing the comm port in the windows device manager to be an unused comm port in the range 1 to 4 and setting the M application to match. Its a fairly fundamental piece of information to know what comm port the USB/serial converter has assigned itself to.

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