I’ve managed to resolve my issue with the Sonos app not finding the system over WiFi after updating iPhone to IOS14. Posting it in case it helps anyone else struggling with this issue. All other solutions I found online ie rebooting router, changing network privacy setting, unplugging speakers, uninstalling and reinstalling app etc all failed for me.
Apparently an additional privacy feature introduced by IOS14 was the use of a private address when connecting your iPhone to a WiFi network. This option can be found by selecting your WiFi network on your phone. I turned this off and the Sonos app immediately worked.