Hello Spaceman,
OK, as of about 10 minutes ago I have a fix but it does involve connecting a 'mesh' network.
Before I switched to Hub5 on Monday this week my Sonos had been connected to a Google mesh/extender network which was fed via a VM Hub3. If this means nothing to you, let me know and I'm happy to explain in detail, it's quite simple. I have had the Google mesh network for years for the sole reason of better extending the wifi around my house. Unbeknown to me the improved stability of the Google product was what kept my Sonos working where the VM Hub3 MIGHT not have.
Upon receipt of the Hub5 I ditched the Google extension as the Hub5 got the wifi around my house better than the extension.
Trying to connect to Sonos reliably to the Hub5 seemed impossible (and might be). I tried factory resets, ethernet cables, every router setting and no joy.
I therefore decided to revert to the prior setup of Google mesh network connected into the Hub5 and revert to that set up for Sonos.
My expectation is that if you repeat the above, yours should work. However, this is where life got really complicated for me because reverting to my previous set up also failed to work! I am explaining this because it threw up another issue which may be relevant to you and is certainly worthy of this public forum.
The Sonos rep that I spoke to this evening, by pure luck, had previously picked up on an issue that Sonos does not list nor admit to, which is that many Google Pixel phones are unable to set up Sonos systems! I use a Google Pixel 6 and he advised my to try with my wife's phone... 5 minutes later everything worked seamlessly.
10 minutes in now and it's still working 🤞