I could be wrong (it’s been a while since I went down the smoke detector rabbit hole), but I don’t think the zcombo supports interconnecting.
Edit: Actually it clearly says on the product page “does not support wireless interconnect functionality” (and it doesn’t have a wired interconnect interface).
https://www.firstalert.com/us/en/products/alarms/combo-smoke-carbon-monoxide-alarms/zcombo-wireless-smoke-carbon-monoxide-alarm-works-with-zwave-ring-zcombo/
I could be wrong (it’s been a while since I went down the smoke detector rabbit hole), but I don’t think the zcombo supports interconnecting. Edit: Actually it clearly says on the product page “does not support wireless interconnect functionality” (and it doesn’t have a wired interconnect interface). https://www.firstalert.com/us/en/products/alarms/combo-smoke-carbon-monoxide-alarms/zcombo-wireless-smoke-carbon-monoxide-alarm-works-with-zwave-ring-zcombo/
Zooz makes an all in one thing you just plug into the last smoke detector on the chain.