'Error establishing a database connection' only on admin

I migrated https://www.thewolfbrothers.com to a new server by-

1 – Downloaded the entire site directory from the old host
2 – Exported the SQL database from the old host
3 – Created a new SQL database and user on the new host
4 – Imported the old SQL database into the new SQL database on the new host
5 – Uploaded the entire site directory to the new host.
6 – Changed the DB_NAME, DB_USER, and DB_PASSWORD in the ‘wp-config.php’ file to the new settings
7 – Changed the nameservers to point to the new site.

All went well. So now the front end is working great. No issues.

But when I try to log in to https://www.thewolfbrothers.com/wp-admin I get a page that says-

Error establishing a database connection.
This either means that the username and password information in your wp-config.php file is incorrect or we can’t contact the database server at mysql.thewolfbrothers.com. This could mean your host’s database server is down.

Are you sure you have the correct username and password?
Are you sure that you have typed the correct hostname?
Are you sure that the database server is running?

But if any of these were not correct, wouldn’t the front end not be working?