This is very dangerous, and I hate to suggest it. I might also be forgetting a step along the way. There is mention of this below method in other posts on this forum from 3-4 years ago. Please dig around the forums before you try this, if you even want to attempt it in the first place.
So I have had this issue with Iris before. They seem to not want to re-pair once they fall off the network for whatever reason. Typically in my case it is when I let batteries sit dead too long in a device.
Do a backup first just in case!
My solution (again, I might be forgetting something along the way) is to edit the Zigbee ID of the problematic device. I usually just change 1-2 digits (I usually just increment the last digit or two, like 0A to 1B), and be sure it isn't the same as another device I have. Be sure to write down the old ID before editing, or remember how you altered that number. Press Save once you have done that.
Somehow that allows the device to pair again. I don't know how or what makes it work, but that fixes something. Anyway after that, you have to take the ID from the original device (before you altered the ID), and place it on the now working device. That makes the database happy, and I think it preserves all automations, if I recall correctly.
Again, do this at your own risk, but I think you could try it by only editing the Zigbee ID, and if it pairs then explore the rest of the procedure.
Examples here: Iris V1 Contact Sensor Reconnect - #26 by m4ha7m4 and [Resolved] C7 - Unable to pair Iris V1 plugs, are Iris V1 devices still compatible with C7? [Yes, they are.] - #5 by neonturbo
