DTN is short for "Do the needful". It is an expression which means "do that which is needed", with the respectful implication that the other party is trusted to understand what needs doing without being given detailed instruction.
Barret: Damnit Nate just DTN already
Nate: Ja Feel
Barret: BET
Chad: Damnit Late Nate, don't listen to Barret, kindly revert ASAP.
Barret: Chad you're such a game snatch
Don’t trust niggas (or nobody can be used either way)
April : he cheated on me I’m so pissed
juni: girl I been told you it’s dtn4l
Do the Needful
You should understand what the needful is, understand the importance of doing it, and have some sense on how to prioritize it
Can you please DTN? This report needs to go out as soon as you are finished.