Hi
These DMEs need programming to retrieve the ISN. If you try to get the ISN using bench mode, you will be asked to update the DME first sometimes.
Connect the CAS on the bench, update the DME, and then try to read the ISN.
Your DME flash status will probably show 'no program in flash' until you update it.
If you are working directly on the car, just update the DME and then get the ISN.