You have to do the Tier 4 mission from the Romulan Cilivilian near the Transporter Pad AGAIN.
Then when you finish it, Hail D'Tan should show up as an interactable inside the final room.
On another note, try to shoot at the Tholian Captain as soon as he spawns.
He seams to have a tendency to walk off and jump off the platform and into lava when he can't find a target, making him unreachable.
Just wondering, what are the dilith rewards for this mission, I have been seeing drastically different numbers, anywhere from 30k to 40k. I personally got 32k I was just wondering if it was a leveled thing, and why.