Your post seems to indicate you have more than one WiFi network - is that the case? Are you using other WiFi access points, or range extenders etc. besides the main router itself? Can you perhaps detail the network setup and then maybe some suggestions can then be made to try to assist you.

Hello Ken, yes I have an extender upstairs to amplify signal and I’m certain that when I had setup Sonos on my old mobile I had only one network and no extender. 
 

I now am not certain which app to use on my iPhone 

I assume you’re using the new Sonos App if it’s the newer Sonos Amp? Anyhow, if the App will let you,  goto Settings (gear icon) then select ‘Manage’ top right of the display you should see a section called ‘Network’ which if you explore will identify the network SSID name in use by the Amp and just ensure that both your mobile and Amp are connected to that WiFi SSID, even if it means temporarily switching off your range extender etc.

Note aswell though, that Sonos do not support WiFi range extenders in their system requirements (see below link) as some (not all) can sometimes ‘mangle’ the connected device MAC addresses, making the devices difficult to discover in some cases.

https://support.sonos.com/en-us/article/sonos-system-requirements

If you do have difficulty getting the Amp to appear in the Sonos App, try temporarily cabling it to the router (if practicable), or perhaps even to the ‘extender’ temporarily and just see if it then shows up in the App.