Avamar: How to successfully restore SQL Databases on the secondary replica in an AlwaysOn cluster settings

• A redirected restore of an Avamar backed up SQL Database is run to the secondary Replica.
• The restore completes.
• The Database on the secondary is in "Restoring state".
• When trying to join to the Availability Group an error is displayed:

Failed to join the database 'ABC' to the availability group 'AG' on the availability replica 'secondary\AG_Sec'.

This procedure is documented in the Avamar SQL Guide under the section "Restore to the original availability group". It reads:
 

When you restore to the original availability group, the restore process can automatically restore the databases on both the primary replica and the secondary replicas. 
You can also restore databases only on the primary replica. 
When you restore a  database only on the primary replica, the corresponding database on the secondary replicas is in a restoring state. 
To restore the databases on the secondary replicas as part of the availability group, manually prepare and restore the databases, and join them to the availability group on the secondary replicas. 
You can also set the databases on a secondary replica online without rejoining them to the availability group by restoring the databases with the RECOVERY recovery operation. 

The secondary is expected to be in a restoring state. The problem is that the Database  'ABC' on the secondary replica 'secondary\AG_Sec' is not in sync with the primary replica. This is because it is missing some log records.

SQL Server AlwaysOn is not able to apply the remaining logs to keep them both synchronized. 

In such a situation the first step is to verify the LSN sequence for Database 'ABC' from both replica nodes.

Run the following query to check this:
 

SELECT msdb.dbo.backupset.database_name,
msdb.dbo.backupset.backup_start_date,
msdb.dbo.backupset.backup_finish_date,
msdb.dbo.backupset.type,
msdb.dbo.backupset.database_backup_lsn,
msdb.dbo.backupset.first_lsn,
msdb.dbo.backupset.last_lsn
FROM msdb.dbo.backupmediafamily
INNER JOIN msdb.dbo.backupset
ON msdb.dbo.backupmediafamily.media_set_id = msdb.dbo.backupset.media_set_id 
WHERE database_name IN ('ABC')
--and msdb.dbo.backupset.backup_finish_date between '1/20/2022' and '12/23/2022'
ORDER BY 
2 DESC,
3 DESC

This gives you a complete backup history detail for ('ABC') for year 2022.
You can customize the date range to meet your needs.
The important point is check if the Primary and Secondary LSN numbers for this Database are in Sync.

To resolve restore the missing log backups on the secondary from the Primary:
Back up and then check to make sure that LSN nunbers in sync before joining the Database to the AlwaysOn Group.