Time to work out a Plan B then, looks like some people will be needing it!
In my case, all my endpoints are RoPieeeXL, which means I can use a Windows PC/Android phone to send music to them via uPnP. In theory that is, never tested this scenario.
EDIT: Instead of using a player other than Roon in an Internet down scenario, one can always look into upgrading your router to one that has a 4G/5G SIM card in it to use as a automatic failover link. Here in the UK, BT are advertising these but they have always been available from many router suppliers. It may not be quick but should at least enable Roon to operate to some degree.