They are not issues I’m seeing with our dual band Home WiFi setup (each band using the same SSID). Do you perhaps have the speakers mentioned setup in different Sonos Households, or do you think there might just be a ‘device discovery’ issue when your speakers are operating on one WiFi band (2.4Ghz, for example) and the mobile controller device/S2 App, are on the other WiFi band (5Ghz) …or vice versa?

If it’s just a device-discovery issue, then you may need to check your router user-manual to see if there any options in its configuration settings that perhaps need enabling to pass the SSDP multicast packets that Sonos uses for discovery between each of the network segments… or maybe speak to the router manufacturers support desk about the issue.

If not, you might have to separate the bands by giving the one band a different SSID/Name and let Sonos just use the 2.4Ghz band, that’s if your router allows for that configuration, or you maybe able to backlist device MAC addresses to force them onto a particular band. Different routers each may have different features, so you will need to investigate further.

Note too, I’m assuming here that your Fritzbox router is your one and only WiFi access point and that you are not using any WiFi extenders etc.