Hi @whopah
Welcome to the Sonos Community!
There is nothing particularly easy about networking, so your frustration is understandable, and common.
One suggestion I’ve seen is that having both mesh and smart hub as DHCP causes an issue, but I can’t make the hub a modem only device.
This is very likely to be the issue. You only want to have one DHCP Server on a network - they essentially define the network, so having two makes things behave very strangely as soon as devices want to talk to each other instead of only to the internet. It is an endless source of annoyance for me that mesh systems do not come equipped to detect another router and configure themselves into Bridge mode accordingly.
As I mention in my Troubleshooting Sonos on WiFi article (which I recommend you have a read of), Google Mesh can be (or was - maybe things have changed by now?) difficult to put into Bridge mode. Although BT’s routers don’t have a “Modem Mode” in the way that Virgin Media’s routers do, you can achieve the same effect by disabling the SmartHub’s DHCP Server and any WiFi broadcasts. With DHCP off, it is important that only the main Google Mesh node (which is, technically, another router) connects to the BT router in any way - no other devices should connect via Ethernet or Wi-Fi, otherwise they will be exposed to the Internet and all the Nasties on it without the protection of a local IP address (though the hardware firewall would still be in place, unlike with a Virgin router in Modem Mode).
After reconfiguring the BT router, please switch it and the main Google node off (even if updating the settings required a soft reboot - allow the soft reboot to complete, then do this hard reboot after). Wait 30 seconds, then turn on the BT router. Once it indicates it has finished booting up (take note of the lights lit before rebooting), turn on the Google node. This should ensure that all client devices get an IP address issued by the Google Mesh rather than by the BT router. I recommend you also reserve IP addresses for all the devices that frequently connect to your network, including Sonos devices.
If you ethernet-wire a Sonos device to the network, please only connect it to the main Google Mesh node, or to the node via a network switch (not to the BT router or to a secondary node). With the BT router’s Wi-Fi disabled, it’s okay to have it and the Google node next to each other, but a wired Sonos unit should be at least 1m away from anything broadcasting WiFi.
I hope this helps.
