Hi, you can use Multiroom to share music from your ND5XS2 to the Uniti, as long as the Uniti is not one of the very earliest models which is unable to support it. Also check that it isn’t running a very old firmware version, which may need to be updated.
If you have music stored on a USB stick connected to the ND5 you can enable Server mode, and this will be discovered in the UPnP input on the Uniti.