FYI it seems to be available on the DIR-835 too, but that is also a 4 year old router.
See link. I imagine it's probably available on a few other models too, but I couldn't find anything recent. My guess is it was cut when the new UI/firmware was designed.
"I work for a tier-2 ISP and I am trying to activate TR69 onto all our live network devices."
What devices do your customers have that you are trying to access? Are they all Dlink? If so, I assume you're not buying them at the local Walmart, and you have some contact with Dlink. If so, this question is better directed at them since they will know what they sold you and what it is capable of and how to integrate it into your network.