By default USB over Network doesn't accept callback connections. You can configure it to accept callback connections automatically or with confirmation.
TCP-port for callback connections
You can configure any TCP-port for callback connections. USB Server must initiate callback connection exactly at this port. so use this option carefully.
Enable UPnP port mapping
If you have a UPnP-compatible network router, you can enable this option for simplified local network access as well as remote access via the Internet.
The program can be configured to accept callback connections only from the people who knows the secret password.