What model iPhone?  All supported iPads and iPhones should work together providing they are all using the same Wifi ssid. 

iPhone 14 Pro and IPad M4 all running latest IOS 18.4

Make sure they are both on the same WiFi.  Some routers use two bands that may or may not crossover, or have a Guest network that cannot access local devices. 

You can use up to 31 devices running the Sonos app if you have one Sonos speaker. (There is an overall limit of 32).

You need to be more specific about whatever your problem is with the iPhone. If it can never find anything then it is likely the “local network” permission missing from the app, or an MDM installed by your employer on your phone that includes a non-tunneling VPN.

I want to be able to alter selection from one IOS device to another. My other iOS device can log on but cannot see current selection. 

“cannot see current selection” - please explain what this means, eg with a screenshot.