By online nas you meant a “local nas that is always online”. yes then create a new learn-dataset and learn snapshots with that. Then use your backup nas as a target for snapshot replication.
If online nas means a VPS on the internet (not in your local lan) - do the opposite. Create your learn-dataset on backupnas (which I believe is locally available) and learn snapshots with that. Then use the VPS-nas as target for snapshot replication (or even better create a local backup-dataset on the local nas and learn replication first.)
The end goal is to familiarize with snapshots, rolling back with cloned snapshot, replication, reverse replication etc. As long as you get hold of the concepts and the replication delays (in case of slow upload to internet) are managable - everything is fine.