The iris 3210-L was I believe a unique device in that it includes both a Zigbee plug, which is what you can turn on and off, and a zwave repeater. The one that you will be able to turn on and off through the app is the Zigbee part.
Because of this unique design, you have to both exclude it from Z wave and reset it for Zigbee before you can add it to a new hub.
From the user manual:
Unplug any devices from the module and unplug from the wall. Now hold the on/off button while plugging it back into the wall. Hold the button until the status LED turns on. Repeat the “Getting Started” steps to rejoin the ZigBee network.
I believe, although I’m not 100% sure, that means you will have to unplug it and then plug it back in again and then you can do the pairing.
Note that to get it to do a Z wave pairing you have it plugged in and you press the button multiple times, some people have said as many as eight, until the light starts blinking.
In order to get it to do a zigbee pairing, you unplug it, hold the button in, and plug it back in while still holding the button. The status light will come on solid (not blinking) and then it’s ready for the Zigbee pairing.
I have no idea at this point if it shows up in the smartthings app as two different devices, it used to.
I believe @nathancu May know more about the process for this model. But this is a unique device and you’re going to have to go through both a zwave and a Zigbee reset, so that’s probably what’s causing the issues that you are seeing.