I bought it used from my local Audio T a couple of years back - I think he said it had been done by Naim and could be reveresed (but I guess it might be in the same way that a vasectomy can..)
Around the time that the Naim DAC was released (2009) Naim updated the CDX2 to add a switch on the back to allow changing between analogue output and digital output to use with the Naim DAC (some people refer to this model as the CDX2.2 but Naim always marketed the updated model as CDX2)
At the same time Naim offered a service whereby owners of older CDX2s without the digital output could send their player back to Naim and have it converted to add a digital output but this conversion disabled the analogue output. It sounds like it was one of these converted players that AudioT sold to you as the serial number of your player indicates it was manufactured in 2007.
It may be possible to send it back to Naim to have the conversion reversed but I would be surprised if it was worth it financially. You would also lose the ability to use it as a digital transport.