FIXED: 1) Make sure you are connected to the same WIFI network on your Mac and iPad, then 2) on you Mac open WIFI Settings, then click on the ellipsis ("...") to drill down further into the WIFI settings, then click "Network Settings". Turn off "Limit IP address tracking" Now go and retry connecting to your iPad using Sidecar via the Displays section of settings. This should now work. You can then turn "Limit IP address tracking" back on. This fixed it for me. Hopefully it will work for you all too! :)