I have 2 paired. If it is still on the iris hub, unpair it and then place the classic app on pairing mode. The thermostat will be in pairing mode. That should work, at least for me it did.
If not, on the thermostat press the menu button. That should bring up the link button on the left side. Press it twice. The yellow led should start blinking. Place the classic app in pairing mode and it should pair up.
For a video google the iris ones.
If you unpair it you should not have to exclude it. But if it doesn’t pair, go to the zwave utilities in the classic app and choose z wave exclusion. Then place the device in pairing mode as described above. In a few seconds a message like device reset will be displayed.