Perhaps this page will help:
https://planet-cnc.com/category/software/
In the Troubleshooting section is this debug example:
Troubleshooting
If inverter will not respond to any spindle gcode command , it indicates that something is not ok.
Type into the MDI window the following text:
=debug = 1
Test the M3, M4 and M5 codes again.
Observe the output window and the printed text.