@ITC,
If relevant, switch off QoS/Airtime Fairness within the router configuration pages and check for 2.4/5Ghz WiFi band isolation. If present, remove the isolation and reboot the router.
Note: Rebooting the router is a worthwhile thing to do in this instance anyway.
Next, see if all your devices show in ‘Settings/System/About My System’ area of the Sonos App. If not. then try a reboot of any missing speaker and if still not showing up after a reboot, maybe try a ‘factory reset’ of that missing device only. Do not reset all your products.
Now re-add the reset speaker to the system using ‘Settings/System/Add Product’ and follow the onscreen instructions to setup and bond/group the speaker(s) as mentioned in the earlier posts.
If the problems still persist after doing these things, then see if wiring just one speaker to the router resolves the issue.. it would be best to wire the ‘Ray’ if you plan to ’bond’ the other speakers to it as it’s not advisable to cable a HT surround speaker without also wiring the main product.
HTH