Hi Lee
I got your phone today and have taken it apart. There appears to be no signs of internal liquid damages apart from the area around the charger/USB connector that has some signs of oxidisation which the repair centre picked up on. The liquid exposure markers within the phone have not been activated so that suggests that there has been no chance that the fault you are facing was as result of any liquid ingress to the main circuits/chips themselves.
The problem you have does seem to be down to some firmware corruption basically the phone has stopped recognising itself as a phone and cannot verify the IMEI. This is not something that we normally see unless there has been some issue occurring at time of firmware being flashed through official update process or in cases where un-official ROMs have been used and something within them has corrupted the firmware itself.
I have tried to re-flash the phone today but this has not proved successful in clearing the issue. We will need to replace the whole main PCB and the re-write the IMEI number to the new circuit board.
I will arrange to get this done at one of our service centres and ensure that they do not confuse matter again by finding the slight corrosion on the USB connection.
I would expect to get the phone sent back to me within about 3 days and can then return back to your home address.
Kind Regards