Cannot setup Era 100: "We couldn't find any sonos products on your mobile device's network..."

Ok - bear with me.

I have an existing setup of Sonos products - Beam, OneSL, One’s and an old Play:1.  Everything is fine.

I have bought two ERA100’s to pair with the beam.

When I attempt to set them up, I get the above message from the sonos app - “We couldn’t find any Sono products on your mobile device’s network so we can’t set up your Era 100. Please try again”.

So far I have:

1. Factory reset both Era 100’s
2. Powered them off for hours, and back on again
3. Powered off and on again the router, and all of the Unifi APs.
4. Moved all Sonos producst to Wifi (I had one wired previously).
5. Made a number of Multicast specific settings on the Router/AP configuration.

Nothing seems to help.

Every so often the message from the app is subtly different, in that it names my SSID, but it’s not consistent.

Weirdly, at one point, one of the Eras did do configuration - I could see it on the Router connected over 5G and it had an IP address, etc.  I could see internet traffic from the Era100. However it never appeared in the app, and after factory resetting it, it never connected again.   I tried forcing it to have an IP rather than a DHCP one, but that didn’t help either - it still wouldn’t appear in the app.

My networking kit is Unifi, and I’ve read quite a few threads and so on on how to configure unifi to get the sonos stuff working, but my current kit is working 100% fine in all rooms, with the exception of the new Eras, which are just sat there blinking their green light forlornly.