Sonos devices and Sonos controllers normally must be on the same LAN.

Maybe move Sonos to your new WiFi (read about issues with that here) and disable your old WiFi. Might want to set your old cable modem to Bridge mode at the same time to simplify things.

I did this a few months ago.  I’m in the UK and have Virgin Media Fibre Broadband.  Their router is rubbish and even with setting up access points there we blackspots in and around my house.

Rather than run two WiFi’s I changed the Router/Modem to run as a Modem only.  Then added the Eero System as the Router and WiFi.  I set the Eero SSID and Password to the same as my, now redundant Virgin wifi settings.  Power cycle of every device, including the the Eero and old Virgin router and all is running well.  Eero now manages all the IP settings and admin of my LAN.

Works very well and I now have WiFi 6 too.

Many thanks all. I was able to put the Eero mesh into Bridge mode, and that solved the issue. 

I had the issue with old SONOS one products. Added TP-Link S7 mesh around the house, new network name. All my old SONOS controllers were wired so i presumed they would pick up the new network, but didn’t. Luckily the TP S7 mesh unit has 3 RJ45 outlets. plugged 2 of the controllers into this and the others then picked up the network.

Had to also set the mesh units to access / extender units 

It took a few hours to sort it out but all seems to be working well now 

This will happen if you put two routers in the same network - devices on one network will no “see” devices on the other network and vice versa.