I'm going to burst your bubble and say that I don't think you will be able to do this.
I'm sure DLNA is only for sharing media between devices on the same home network (e.g. a mobile phone and television), and as mentioned works by having one device serving media to a second device that knows how to render (play) it. You can't use it to send any random file to any random device.
I've never seen it mentioned that DLNA can be used to access media remotely, and in MusicBee's case anyway the plugin is to send the media, not receive it.
As for remotely accessing the files on your NAS, this isn't as simple as just mapping a folder over the internet and pointing MusicBee at it. NAS servers do provide remote access to your files, but it's via WebDAV or FTP or possibly some other proprietary service. That's why each one provides their own mobile apps for accessing the files, images and media etc. on their devices. I haven't seen any way of just accessing files on a NAS via a mapped drive in Windows Explorer, which is what you'd need for MusicBee to access them. This doesn't mean it's impossible, but I don't think it will be easy. Even if you did find a way, I also question what kind of experience that would provide. MusicBee could read a lot of the track details from its locally stored database, but if it had to access the original file for any reason it would have to do so over the internet. If you're displaying a list of 30+ tracks and it had to access each one to read the bitrate or duration or whatever, I imagine that would be unusable. I don't know if that's how MusicBee works, but I figure it must be reading some of those details from disk some of the time.
Given that your question isn't really about how to get MusicBee to access NAS files over the internet, but how to get Windows to access them, you'd have better luck asking in a Windows networky / internetty type of forum, or possibly see what responses were given to similar questions on Buffalo's forum, which I assume is a thing that exists.
Bee excellent to each other...