Back 2 U is the 3rd track from NCT 127’s 2nd mini album, “LIMITLESS”. It was teased previously in upcoming teaser snippets from late December 2016 – early 2017.

Here is Yuta’s 2nd “LIMITLESS” teaser, of which a snippet of the song is included.
https://www.youtube.com/watch?v=zdW2LPcsMaU