Greetings, old Win guy
You know way more about Reaper than I do, but since I'm sitting at a Mac and no one else has responded, random thoughts. Damn. that's an old synth - is it possibly broken? I'm sure you've checked this, but under prefs/audio devices/midi devices, is it in midi devices to make available (selectable as track output)? Found this link to an
owner's manual, but very little info on midi connectivity. Sorry, that's all I've got.