friends of mine also had the problem that the device did not want to join but a device reset (by pressing all three buttons until the display counts to 10) solved it for them. one of their devices needed a couple of retries though…
you have to search for a sensor (not switch) using the phoscon app. even after a successful join you won’t see the device in the app but they will show up in your mesh when using vnc-viewer.
you also need some more steps as explained here to have them show up as HVAC devices: [SOLVED] ConBee II and Eurotronic Spirit Zigbee integration