In DAG environments, you can simply reseed the Content Index State from a healthy node in the cluster. I will show you how to perform this in this blog post.
Here we have two databases in a DAG cluster which have the index status in a failed state:
To manually update the ContentIndexState from a healthy node simply run the following command:
Update-MailboxDatabaseCopy "database\server" -CatalogOnly
In my case the database I want to update was "CCEX2-DB-02" on server "CCEX1" so I ran:
Update-MailboxDatabaseCopy "CCEX2-DB-02\CCEX1" -CatalogOnly
After running the command you can see we brought the index for the database back to a healthy state.
Repeat this process for any other databases with a failed content index.