Connecting to SQL Express Server 2017 with SQL Server Authentication Remotely
Typically, a default instance will use port 1433, and a named instance will use an arbitrary port in the 49152 to 65535 range. Because SQL Express uses a named instance (even if it is the first and only instance on a server), it will grab an arbitrary high port. So make sure that TCP port 50207 is allowed from your second machine to Desktop. And also open UDP 1434, in order for the SQL Browser service to do its job.