What streamer and preamp are you using? The +/- volume control is used when ‘system automation’ is enabled and an RCA to 3.5mm jack cable connects the two boxes (as opposed to the slider control for all-in-one boxes) so maybe you haven’t set this up? This assumes you are not using the variable volume mode, and have the streamer in fixed volume mode as recommended for best sound quality.
Back in the day when Naim produced proper manuals, this explains how to set it up:
The cable you need is a very cheap, thin one that connects the streamer to the preamp, so it’s not a big deal to set it up. Having said that, you can still use the streamer remote to control the preamp volume without it, just not the app, so if you’re happy with that, just leave it.
This still controls the analogue volume pot on the pre, so you are not resorting to a potentially inferior digital volume control.
The volume control is not for individual tracks - it’s a controller for the pre-amp’s manual volume, via the system automation.
As @ChrisSU noted - all you need is a cheap (EUR 2 cable) and you’re set.
My dealer gave me one when I bought my 282, which was very generous of him, but it was too long, so the 65p purchase was an attempt to tidy up the cable dressing mess. I have now ditched both of them, and just use the volume controls, as I can reach it from the sofa, and it’s easier than the buttons on the app.
I connected an old cable I had to hand I used to use to connect an iPhone to a portable speaker, nothing fancy as others point out. In my case I have an ND5 XS 2 connected to a SN2 so there is no RCU or front panel display on the streamer, this just means I don’t need to use the SN2 NARCOM to handle volume control and can do it all in one place via the Naim iOS app. I would point out that I tend to tap the + rather than press and hold it as it tends to increase the volume fairly quickly so can catch you out.