I've worked briefly with RtMidi and it is, as you stated, a rather complete library. However, when working with MIDI devices, you have to take the specific issues of this type of devices into account.
Another thing is that cheap USB-MIDI adapters often simply refuse to work with some applications "just because". They are very simple inside, yet apparently chinese manufacturers are still able to make malfunctioning devices.
If you're using unmodified, example code, and it's not showing your device, I'll first ensure all other apps that might be using it are closed, then try again. Creating a virtual interface is also a nice idea; I personally use freeware LoopBe1 for it. If it will show the virtual, but not the real interface, then the problem is with device itself. In that case you might want to experiment with different drivers or simply plugging it into a different port.
If it doesn't show any device, neither real nor virtual, then I strongly suspect there's something wrong in the way you build/run the application.