In what way do you power off your Sonos speakers? Do you cut the power? If so: Sonos advises against this -  and the speakers go into a low power mode three minutes after last being used anyway. And some routers get confused when all speakers power up at once.

You could try to reserve ip-adresses for you Sons devices in your router - it saves the confusion I mention above.

Thanks for the advice. Yes, I do cut the power at bedtime (well, Alexa does it when I ask). I guess I am a bit old fashioned in preferring not to have unnecessary appliances powered on at night, but if that's how Sonos speakers are designed to operate then that's what I shall do.

Thanks again.