If your iPhone is presenting a different MAC address to the app, then the app may not link to the discs. Turn off private addressing (MAC randomisation) so that the true hardware MAC is being presented. Leave it that way.
You may need to re-install the app so that it associates with the iPhone hardware MAC address. See if it helps.
I've turned off private addressing on the wifi information, and i even turned off private relay, but still getting the same.
Did you re-install the app?
Which BT Home Hub version are you using, I assume you are a BT Broadband customer, as this is a BT broadband forum?
It's not a BT Home Hub, it's a Vodafone one (I used to be a BT Broadband customer, but not for some time now). And it was working perfectly well when I last checked a few weeks ago. And like I said, the network is working as all of my devices in the house are connected to it. It's just the app that's stopped recognising the Whole Home Wifi discs.
And yes, I did reinstall the app.
If it had been a home hub I would have suggested a factory reset of the home hub. As you are with Vodaphone I cannot really suggest anything, but I suppose you could always factory reset the Vodaphone router, just in case its having internal routing issues. Remember the app has to route to the primary disk via the router, so a loss of internal routing will stop it seeing the disk.
I suppose you could always look on the Vodaphone user forum to see if anyone is using the same disk system.
Just adding some support here that I've recently discovered the exact same problem:
1. The Whole Home App can no longer detect the discs, even though it's on the wifi network. It was working 2-3 weeks ago.
2. My whole home network continues to work completely fine with 4 discs
3. I use the discs for my wifi but the router is a virgin hub
I'm on android, and I've also discovered that the mybtdevice.home address does not resolve either.
I assume it's an update on the app that has caused this...
Update from me...
I phoned the BT helpline 1st line support and they couldn't get me to do anything I hadn't already done, and they were unable to tell me the problem, but they gave me the email email@example.com to send my logs to, who are the 2nd line support who can look into it in more detail. I've yet to do this, but will be doing today.
They also told me to use http://btwhole.home to access my disc network from a browser, which has worked for me. So at least I can manage my network via my laptop just not from the app. So try this ChrisSSS.
Edit: Oh, and you can export your logs from http://btwhole.home to send to 2nd line support if you want to do the same.