I have reached Tier V on New Romulus and have acquired the mission to go to the Underground Ruins and then to signal D'Tan when the final room is clear. I have gone to Underground Ruins Transport point but cant seem to find out where to go from there? Can someone please help.

Once you complete the Underground Ruins mission, stand in the last room (the one where you fought the Tholian Captain and tried to shut down the power plant). You should have a prompt to signal D'Tan if you have "Secrets of the Ancients" in progress.

Just to clarify what Psychoticvulcan said, you have to actually clear the tier 4 mission again. All those Tholians. Hope you kept your EV suits.

There is a bug I ran across. I did the mission, but no 'contact d'tan' at the end. THAT mission popped up AFTER I left the ruins and I had to run it again after the cooldown the next day.

This is something I kind of find to be a nuisance as well. I had the tier 3 mission in the caves, so I went in and cleared the basic NPC mission, and didn't realize until the mission was turned in and I had already left the caves that I hadn't yet completed the tier 3 mission itself. I had to wait for the 24 hour cooldown to go back into the caves because you can't enter unless you're on the basic NPC mission.

Many Many thanks to everyone for your replies.

I didnt realise I had to run the Tier 4 Lava Tubes again as it didnt indicate that, so to everyone once again thank you very much, problem solved