Is your speaker there if you log into the desktop Sonos app on Windows or Mac?

https://support.sonos.com/en-gb/downloads

Yes, it is showing on the desktop Sonos app but not the phone app.

Have you updated the app to this week’s latest version, and then run the firmware update at:

Settings / Manage / System Updates / Check for Updates

It could help.

But if you’ve done that already, then another thing could be:

• Router and speaker off at the wall.
• Force close app, phone off.
• Restart router until it is fully online.
• Turn speaker back on at the wall, let it connect (white light).
• Turn your phone back on, and reopen the Sonos app.