Also, for anyone reading this in the future having the same issues as me with getting the Dimmer 2 to be added to Smartthings I just wanted to detail how I resolved it.
First things first, as I think @siwilson suggested earlier in the thread, when I thought I was resetting the Dimmer 2 it wasn’t actually resetting. I realised this because after doing what I thought was a reset I checked the hub events log in the IDE and there was an entry on there suggesting it couldn’t exclude the Dimmer 2 as part of the reset and I should use the exclusion utility to force it instead. As a result, the reset process didn’t happen.
So I then excluded the Dimmer 2 using the exclusion utility on the ST app and pressing the B button 3 times. That gave me a message that a device was excluded. I then quickly used the B button to toggle to the Yellow reset mode and activated that. This time the reset worked because the device had already been excluded and also you will know it is going through the reset process because the Green light blinks while it’s happening. Once it ends, the light goes Red for a few seconds and then turns off.
The dimmer 2 is now excluded and reset. I checked my hub events and they confirm that. I then put my ST app in pairing mode and triple clicked the switch but unfortunately still no luck in the app - the search process just goes on endlessly so I ended it after a few minutes wait. However, when I checked my hub events it showed that shortly after I started the inclusion search there were three further entries showing “device setup”, “zw join”, “device configuration” and “ready”. The “zwjoin” entry included the ID that the device had been assigned in the ‘Value’ column. However, there is no device with that ID in my devices list in the IDE. Why this background inclusion is happening without any feedback on the app or adding to IDE I have no idea but I then followed @siwilson’s suggestion to manually create a device via the IDE with the exact same ID and assign it a DTH used by other working and already included Dimmer 2s.
I then saved that and then turned the light on using the switch. As a check, I went into the properties of this manually created device in my IDE and checked it’s events. There I could see the event for when I turned the light on and power stats. This proved the device I manually created is in fact the dimmer 2 at the switch. The device also then appears in my ST app and I can turn it on and off via the app and all is good.
It would be useful if others might ever be able to answer what causes this background inclusion but I hope this is useful to others and thanks all on this thread especially @siwilson for helping me out. I’ve never used the hub events log but it was so useful in tracking what was going on at each step and getting clues for what might be the issue.