This is almost certainly a router or network setup issue.
When you say you have two Wi-Fi networks with different names are these provided by the same device or separate devices? If two devices you may have created a subnet or are operating two routers in router mode, so are suffering from double NAT, which will give the error you see.
My wife has a iPhone 12 and the Naim App finds and operates my Qb and Atom without any issue. Delete the App and re-install to see if this helps. Maybe you failed to acknowledge the pop up request to allow network access.
Depending on how your network is configured, if you have the 2.4 and 5GHz networks on separate SSIDs I can see that this might cause issues when the iPhone is on a different network to the Muso.