On a Router object, you can call the createPlainTransport method. The parameter of that method is PlainTransportOptions, which allows you to give the port the PlainTransport explicitly is listening on.
You can also set the option rtcpMux to false, which indicate that the RTCP and the RTP traffic should flow on two different ports, and in that regards the PlainTransport is created with rtcpTuple.
The rtcpTuple will have the localPort attribute for receiving RTCP traffic from the remote endpoint, but that is out of control of the creation, meanwhile the port for RTP is not.
Is there a specific reason to deny the setting for rtcpPort in the PlainTransportOptinos?