If you do a quick search there are several threads on here covering this

 

just tried, didn't really find much

 

Some links below may be worth a read.

If I was you I would call up the parts department at your local Audi dealer so they can look it up on their system in case any part has been updated.

https://www.ttforum.co.uk/threads/rear-diff-question.2034840/?post_id=9645963#post-9645963

https://www.ttforum.co.uk/threads/differential-failure-tt-mk3.2027416/?post_id=9592728#post-9592728

https://www.ttforum.co.uk/threads/rear-differential-issues.2032448/?post_id=9624421#post-9624421

 

This is not much help now, sorry. I had same with diff failure in my 2016 Mk3 TT quattro. Mine failed at 40,000, full Audi service history, dealer paid for new part, filter was blocked. I had new diff serviced at 20,000, filter as new and totally clean. Audi Service interval is too long.