App is not designed for remote access, although the companion app FileBrowser can be set up for remote access. I never have done this since I have other ways to access remotely (e.g. an app for my Asus router which enables me to access router, and network, and a drive connected to router with media on it.) In addition, I also have a WD My Passport Wireless drive that my iOS devices can access to play the media on it. In fact, I now have two music sources connected to MusicStreamer; my My Cloud network NAS and the wireless drive. Cool.
Although. right now. I am "starting over" to populate the MusicStreamer app. After experimenting with it a while, I have un-installed it and re-installed it, so currently it is re scanning my NAS, after which I will add my wireless drive to the app and scan it again.
BTW, once I started over, I was more patient, and when the app first scanned the network for sources, it found my NAS after a minute or so; in fact ,it found all six devices shared on my network. Easier to set up NAS; no typing in IP or anything.
So, I suggest the same for you: Uninstall and reinstall the app, when it begins, be patient and go through guide for tour, and then have app scan the network for shared devices. It will find them all, including your NAS. You may need to slide list left or right to see it all. BTW, no port forwarding necessary. The less you do, the better!
Good luck getting it running.