Hi @Marin ,
Check as following:
- Make sure that you can ping your SQL server IP from the SharePoint front end server.
ping Server IP - Make sure that the SQL Server Port is opened (the default port: 1433) by performing a telnet of the DB server IP with the SQL Server port in WFE server.
telnet "the SQL Server IP" 1433 - Make sure the App Pool account for Central Administration has access to the SP Config DB.
- Make sure that the SharePoint Configuration database is not in recovery mode.

- Make sure that all SharePoint servers within the farm are joined properly and working healthy!

Once everything is fine and you are still having issues, restart the SQL host service on the SQL server. Once the service is restarted you will need to reboot Central Admin and then your front end servers
More information for your reference:
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