It appears that some change to Spark's wireless modem router (the rural broadband offering), now disables Chromecast devices from talking to eachother via the Google Home app.
I have a customer with two of the newer (round) Chromecasts that used to work fine with the same modem in the near past.
Despite resetting everything and then doing it all again while on the phone with Google's Chromecast help person, I have had to admit defeat!
She agreed that it is the "AP Isolation" setting that needs to be disabled. (also known as 'Client Isolation' or 'Guest Mode')
Google was willing to ring Huawei to get it sorted (ambitious!), as Huawei warn you in their introductory support phone message that modems are configured by the retailers (such as Spark),
Spark, in turn was only able to send me the generic How-to documentation - which confirmed that I had been unable to find this setting in their firmware on this modem - as its not there!
Go back to copper based ADSL: No
Get Spark to change their firmware: unlikely
Add an additional wifi access point for the home setup...
Any other ideas?