Is the iPhone on the same network subnet as your Nucleus?
If so, typically (when using a netmask of 255.255.255.0 as is the default in most home networks), the first three (of the four) numbers in the ip address should be same.
I did not supply an ip address. I supplied a netmask - which is a setting in your router that defines the size of your local subnet. I only put it there because the following statement (first three numbers of the ip address define the subnet) is only true when the netmask is set to 255.255.255.x (where x is usually, but not always, 0).
In order to connect, an iPhone (or any other device in the Roon system) must be on the same subnet as the Roon Server.
Thus, if your router is accessed on 192.168.w.x (and the netmask is set to 255.255.255.0), you Roon server and your iPhone should be on the same subnet and will thus have ip addresses 192.168.w.y and 192.168.w.z respectively where ‘w’ is a number between 0 and 255 and x,y and z are (different) numbers in the range 1 to 254.
Welcome to the forum. Can you please confirm your network setup (model/manufacturer of all devices) and specify how your Nucleus is connected to the network? If you navigate to your iOS settings → Roon, is Local Network Access toggled on?