As a next step, could you please try connecting to your Nucleus directly by its IP address from the Roon app?
If that does not work, please enter the Nucleus IP address directly into a web browser on your phone or computer and let us know whether the ROCK Web UI page is accessible.
Also, please check the IP addresses of both your iPhone and Nucleus and let us know whether they look similar. For example, they should usually be on the same subnet, like:
iPhone: 192.168.1.xx
Nucleus: 192.168.1.xx
If the addresses look very different, such as one being 192.168.0.xx and the other 192.168.1.xx, they may be on different networks, which could prevent the app from finding the Nucleus.