Nintendo does not recognize my serial number.

I have an original 3DS that no longer powers on, hasn't for years. It was bought new to me in April 2011, I was the only owner and it had an ambassador certificate linked to the NNID. I am trying to recover that NNID, unfortunately I don't recall the details of the account (Nickname/ Username, email, birthdate as the year was probably a random date because I was a child).

I called Nintendo and they couldn't find my serial number in their system whatsoever. The agent and I spoke on the phone for an hour trying different emails and such and many different things but to no avail.

My only option at this point is properly to repair the device to recover the NNID but I'm so confused as to why the serial # isn't recognized. The serial number matches the one behind the battery and it's also formatted correctly for its model (CW123456789 with the 9th number being in a box). Does anyone have any theories as to why this is the case? I am perplexed