A network-related or instance-specific error has occurred while establishing a connection to SQL Server

We have 4 servers, three of which are the main servers that contain SQL Server databases, and we are going to the mobile app on server 4 so that our users can connect to their database on their phones. Server 4 has an intermediary role, but now the problem we have is that we can not register data through Server 4 and it gives an error. I checked the firewall and SQL Server security settings, but this problem still persists and I get this error:

Microsoft.Data.SqlClient.SqlException (0x80131904): A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SNI_PN11, error: 26 - Error Locating Server / Instance Specified)

I also checked the remote connection settings and it is active

Please check whether you can you ping other servers from server 4 to ensure there is no net work level problem. Then you need to allow remote connection in all those servers.

Below link might be able to give you more clarity on this issue:

Why am I getting "Cannot Connect to Server - A network-related or instance-specific error"?