Sonos One - Both speakers not connecting to network (wired and wireless)

I bought a pair of Sonos Ones in September 2020 and they have been on a shelf, working flawlessly, until a few days ago.  I opened the Sonos app on my Apple iPhone to play some music and the app reported that it could not find the speakers. There had been no network changes to the WiFi network they connected to and other devices (including the iPhone) were still connecting to the WiFi network fine.

Following the prompts on the iPhone app to reconnect the speakers to the network (using a Bluetooth connection between the phone and the speakers) progressed well until after the WiFi network password was entered, after which the iPhone app immediately reported that the Sonos One failed to connect the network.  Both speakers behaved exactly the same.  I tried factory reset (holding down the connect button while powering on the speaker), but the result is the same.

I moved both speakers off the shelf to my desk where I have wired ethernet and tried using a wired connection on one of the speakers. This also failed to connect the speaker to my home network and I can’t see any evidence of the speaker acquiring an IP via DHCP from my router.  The ethernet cable functions perfectly when plugged into my laptop, so there is no problem with the cabled connection to my router.

I seem to remember that the speakers where set up to automatically apply firmware/software updates and I did allow them access to the internet on my router’s firewall.  I am wondering if they automatically downloaded an update which failed in some way and broke network connectivity?

How do I troubleshoot this issue?  Any help hugely appreciated.  I am missing my music. 😢