Then multiroom should work, although the latest firmware version is 4.8, and while multiroom should work on 4.7 I would run the update anyway.
Can you confirm that you are looking in the right place to enable multiroom:
Is multiroom enabled in the ND5XS Input Settings menu?
In the Naim app, when you open the Play Queue screen do you see a button in the bottom R corner consisting of four little squares? Tap that to select multiroom controls.
Some Hi-Res formats can’t be multiroomed. To be sure, select a regular 16/44.1 file which should work.