The on-board LED should be blinking during normal operation when not-connected over BLE.
The RSL10 enters deep sleep when LED is off, so you cannot debug at those times.
The debugger can reset and re-flash the board even if RSL10 was in sleep mode.
I would also double check the orientation of the 10-pin cable from debugger, some adapters allow you to plug in the wrong end of it.
You can disable sleep modes in the
RTE/Board_Support/onsemi_smartshot_config.h file for debugging (see
Configuration section of the readme file).
It is also possible that the original FW used older version of FOTA and you also need to re-flash the FOTA binary as they are linked together. Instructions on how to get complete Flash image are in the
Generating initial Flash image part of readme. You need to Flash this once and then you can use normal debug sessions as long as you do not update the RSL10 CMSIS-Pack version.