Hello, sorry for the delayed response. Your answer somewhat helps but not entirely.
I was not familiar with UPnP so I had to look into it, I’m unsure how this would be different than if I were to stream off my NAS through its app. For some reason I’ve had problems streaming (either from the NAS or to Chromecast) and I believe it has to do with my building as all my hardware is fairly new. Even my phone reception is poor to non-existent depending on where I stand in my unit though the provider says there is a tower very close to me. As convenient as sending music wirelessly would be, I don’t have great confidence that it would work well where I live.
On top of that - and more importantly - my desire to run a hard drive off the back of the Matrix unit is for redundancy purposes. I have confidence in my NAS but want the added protection of files on an additional piece of hardware.
The matrix will allow you to plug in USB storage for music playback.
I'm more curious to your wireless streaming issue. How old is your NAS or your router?
You should be using WiFi within the house so your phone signal shouldn't matter.
I recently replaced my router that's coming to 7 years with a new one, along with new Ethernet cables all round. It definitely made a difference for my boy when he's playing games and chatting at the same time!