Now you are able to run readonly query on the secondary database:
If you try modifying data on the secondary database, it will be fail:
If you did NOT enabled the "Disconnect users in the database when restoring backups" option, then the Restore Job cannot restore transaction log backups to the secondary database if there are users connected to that database. Transaction log backups will accumulate until there are no user connections to the database.
Any data modifications in the primary database will be transferred to the secondary database.
Trick: in order to prevent the readonly query blocks the restore job, you can specify the connection to USE another database, e.g. master, and query the secondary database tables using three-part-name.