A further update
I still can't get the Roku 4k Streaming Stick 3810X to respond to remote controls seeking to pair.
Usually the method is to get green button flashing by holding the button in the battery compartment for 5 seconds and booting up the Roku, the pairing would then happen automatically.  
The remotes have been tested on a different Roku and pair with no problem.
I have made some progress on the App side by changing the settings on my Router, I used the mac address of the dead Roku Streaming Stick 3810X and set a static IP for it and I set the router to NOT isolate wi-fi clients.  Isolating clients is a security measure.
This enabled me to find the dead Roku Streaming Stick 3810X, I was able to confirm an update happened yesterday, which was version 13 build 24028.  I checked but it says this is up to date.
I also investigated the remotes, it still had the two voice remotes that I previously connected to it and that worked before the update.  I drilled down those voice remotes and it said they were asleep, but it would not respond to them.
So I manually used the mobile app to delete the existing voice remotes and used mobile app to manually pair, this got my remote paired, but it still does not respond to pairing on boot (the usual way if you have no remote).
I was also able to add an old gaming remote although it will not control the TV.  
The Roku 4k Streaming Stick 3810X is still damaged by the firmware, it will not pair with remotes in the usual way.  I guess I will have to wait for a new update to see if they fix this damage.
Whilst I have this working for the moment it is not fully operational, so I do not want to close this thread until Roku fixes this.  I will put further progress in this thread. 
I have reset my router to isolate clients as this is my preferred security state.