Heartbeat failed to connect to standby

If Primary database alert log is showing error message similar to the following,

PING[ARC1]: Heartbeat failed to connect to standby 'standby-db'. Error is 16058.

it means that primary database is not able to ship redo log data to
standby database because standby database instance is not mounted; which is required for shipping the redo log data to standby site.

To avoid this error and keep standby in-sync with primary, make sure that standby database instance is in mount state and is accessible from the primary database.