a lost serial connection can have some trivial causes. Quoting from existing threads:
In case you cannot find the cause after checking all these, you have two more options:
Run the spindle manually, without CNC Controller ModBus control
You will have to alter the two options for “run/stop” and for “speed control” in the VFD to get back to manual mode. As long as they are set to ModBus control, the “run/stop” and “speed” control on the VFD front panel won’t become functional.
or
control the “run/stop” and the “speed” of the VFD via the analog spindle control of Onefinity Controller (pin 15 “tool-enable” and pin 17 “PWM”).