If you still have the older iPhone can you compare their settings to see if some setting is differently in the new iPhone?

For example, local network access enabled for Sonos app. In your Wi-Fi network settings, the Private Wi-Fi Address. 
 

Although, I have seen people with iOS 18.1 writing that they have problems getting the speakers recognized and set up.

I don’t remember if it is still possible to set up speakers with the windows or Mac desktop app but I’d look into that if you already haven’t. 
https://support.sonos.com/en/downloads

Thanks for the reply. I called Sonos support. They indicated that my router had 2 different IP address for the Sonos components that was not allowing the Sonos components to connect to the router. System was operating on my phone before we concluded the call.