Both Maestro and Solo:2 will be visible in the devices list in Spotify. It can take some time from opening the Spotify app until all devices are shown in the devices list. Make sure that both the Maestro and any Solo:2 speakers are powered on and visible in the IXION app through your local network. You could also try to restart the Maestro if the mentioned devices are still not showing.
The Solo:2 speakers require the Maestro to be powered on to be visible in the devices list.
Spotify also have a function that allows playback on Maestro even if you are not on the same local network. On units where its possible to log in with your Spotify account you will see that device in the Devices list. On units like Maestro and Solo:2 the user have to play from the local network before the unit becomes visible outside the local network. This because Maestro have no login option for Spotify.