I assume you’re using the new Sonos App if it’s the newer Sonos Amp? Anyhow, if the App will let you, goto Settings (gear icon) then select ‘Manage’ top right of the display you should see a section called ‘Network’ which if you explore will identify the network SSID name in use by the Amp and just ensure that both your mobile and Amp are connected to that WiFi SSID, even if it means temporarily switching off your range extender etc.
Note aswell though, that Sonos do not support WiFi range extenders in their system requirements (see below link) as some (not all) can sometimes ‘mangle’ the connected device MAC addresses, making the devices difficult to discover in some cases.
https://support.sonos.com/en-us/article/sonos-system-requirements