When you are running Renoise as ReWire master, Renoise can only show up and use MIDI ports that the ReWire slaves do publish. For example you’ll see Reasons synths as ReWire ports.
When running Renoise as ReWire slave, Renoise currently publishes no MIDI ports to the master. Please see https://forum.renoise.com/t/using-ableton-live-instruments-in-renoise/24898 on why/how/whats the problem…