It may have the same WiFi SSID, but it is a different network and will have a different BSSID (Basic Service Set Identifier), but even that may not always be the cause of your entire issue, Sonos controllers use SSDP Multicasting to discover its devices on the LAN subnet and some (not all) extenders, or access points, can sometimes have difficulty with the multicast packets and may drop them, causing the device discovery to fail. 

Sonos does not officially support some network setups as mentioned below. This taken from their System Requirements…

Unsupported network setups and devices

• Wireless internet connections such as satellite, mobile hotspots, or LTE routers
• Guest networks or networks that use a portal login page
• Networks using wireless range extenders
• Ethernet over Power (EOP) devices
• WPA/WPA2 Enterprise

If you mean it disappears from the app when your phone switches between 2.4 and 5GHz, then get a smarter router. Your current router is poorly implemented.