You’re right in that mqtt and mqtts are not officially recognized iana URI schemes. Some client libraries, such as for dotNet and Node use this as a convention though.
It sounds like you’re using a Java MQTT library, most of which require the use of the
tcp scheme. For example, see the docs for the HiveMQ Java library
You would use port
18103 for non-SSL which would not be very secure. Use port
28103 to connect over ssl, and don’t forget you’ll have to configure SSL options in the MQTT client before you connect.
Hope that helps!