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Virgin Media Warns UK Broadband Users Not to Switch Routers Off at Night - ISPreview UK
(www.ispreview.co.uk)
This is a most excellent place for technology news and articles.
Have the router ask the server if there's an update available when turned on. If none, proceed as usual; if there is, force the update, regardless of the time of the day. Problem solved.
Of course, for that you need to acknowledge that you violated the "ask, don't be an assumer" rule, instead of bossing customers around with "golden rules". You won't change their silly and pointless habits anyway.