When you do not have a cellular or wifi connection, you can still take attendance for existing classes and practices.
Take attendance like normal.
You will see a sync icon by the practice or class. When you regain a data connection, it will automatically sync, and the icon will disappear.
If the icon does not disappear automatically, tap it to activate syncing manually.
If another admin also attended the same class or practice, you will see any conflicts and have the opportunity to resolve them.