Unplug your speakers for a couple of minutes and reboot your router and phone.

Maybe try reserving/fixing all your Sonos speakers IP addresses in the local routers DHCP reservation table, so that the router assigns them the same IP address and with any luck your App will know where to find your speaker on the network.

This issue might also be software running on your mobile controller device, preventing the discovery of your speakers, such as Firewall, VPN client software etc.

If you have more than one WiFi access point and are using WiFi extenders across the network etc. this too may cause this type of issue, as the mobile controller connected to one WiFi AP might not be able to get through to the speaker(s) connected to a different WiFi AP with the multicast broadcast ‘device-discovery’ packets.