I have combed through the Solarman, Solarman Business and Deye Cloud apps and I can not work out a way to enable a notification when grid power drops. Seems like a basic but critical feature that is available on the Sunsynk app. Any advice? 

Interestingly the feature was only added on SS a few months ago

But you are correct - its not available on Solarman. You can infer this through checking for the Grid Frequency or Voltage though