Helo @jos0,
the name "holding register" doesn't mean the value is stored in the non-volatile memory. This is just a type of the data store which comes from the Modbus naming convention, which means it can be read and written into, as oppose to the Input register, which can be only read.
You can save the current value of the counter register with the saving of the current values to internal flash. This save can be initiated by e.g. setting the coil named "Save current configuration and use on startup" which has to be called on the appropriate group.
We do not recommend to do it very often as this will corrupt the internal memory. Try solving the problem in the program instead.
Best regards,
Martin