To keep your data up to date in the mysms cloud, there is a complete sync every 24 hours. In case the notification “Messages 24h not synced” appears, mysms was not able to establish a connection to your smartphone. Reasons for that can be a disabled data connection, task killer or a proxy (using Wi-Fi mode). You could try to perform a manual sync in the mysms settings on your phone (Sidebar - Cloud Synchronization - Manually Sync). After the sync was successful and has completely finished, the notification should disappear after refreshing the web app.