Fitness trackers may continue working and showing data even once a connection to the smartphone or internet is lost. However, in order to get data in the cloud, they need to be synchronised with an app. Some of them need to be synced manually - an app needs be opened.
If an app has not been opened for a day or more, then there will be no data in the cloud for that time period. Once opened again, most likely an app will sync automatically and data will be exported to the manufacturer cloud and the data could be transmitted to Conrad Connect.Unfortunately, data will not fill the missing days, so it's recommended to check the app daily. Please note that cloud synchronisation requires an internet connection.
However, Google Fit service doesn't have this issue and updates automatically as long as auto sync function is enabled. This function is enabled by default.
It might be also possible to link a fitness tracker to the google fit service and authenticate google fit with Conrad Connect, so device may synchronise automatically and data will always be available on Conrad Connect.