It sounds like you you want hear music coming out of your phone, but your music files are stored on a NAS. You also want the music files to be seen by your Arcam unit. If you want to cast music from the phone to another device, that is not what the text below is about!
You need a component on the LAN that has network file access to the NAS unit, and that will also work as a DLNA/UPnP server to other devices. The other "devices", include a phone, tablet, PC, Arcam etc., and device will then need to run client software that can 'see' the DLNA server on your LAN for playback of the music, and have means to control viewing and presenting music files to the playback system - the controller and playback system might be combined. Some NAS units happen to run a DLNA server internally but most do not. I have an old Buffalo unit that does - it serves videos and/or music to every device in my house.
One way is to use a device like a Raspberry Pi to act as a DLNA server, which will connect over WiFi to your NAS. Something like Volumio is a popular way to make a Pi into a DLNA server.
In fact, you can run a DLNA server on many compute platforms, and they can be the cheapest old PC you can get your hands on. If you run Linux on an old PC, you can run Volumio on it as well many similar apps to make a DLNA server. It's functionally the same as using a Pi. You can use a Windows PC, and run something like Universal Media Server on it (I use it myself, and it's fine, and unlike Plex etc, it's very simple and doesn't report data about you back to base!). Again there are many free DLNA servers you can run on an old Win PC....
Then you could run something like VLC on your phone. Once setup right, it will discover your DLNA server on the LAN, and let you playback music and videos that live on your NAS. You can also run Foobar2000 just for music, on your phone, and that app will also do gapless playback (unlike VLC). You can set up playlists on the NAS and play those back through DLNA..
I think your Arcam will discover the DLNA server you have setup, and there will be an app to control it (or use a browser) and it will presumably then play into you amp and speakers. I believe that device is an example of DLNA client (or renderer if you prefer the standard terms), but which also needs a controller (browser or separate app), unlike VLC or Foobar on a phone.
Apologies if I misunderstand what you are trying to do!