Need mission to get into caves. There is a Romulan Civilian that gives the mission by the teleport pad. The Tier 5 cutscene mission requires the mission unlocked at Tier 4 which is at the Underground Ruins Teleport Pad.
All Romulan tier missions except 1 and 4 require you to ask a civilian for a second mission to be able to enter the relevant area.
And yes, it would be nice if the tier missions instructed players to go to the civilian first, if only so people would stop asking how to get in all the time.
The need for environment suits in the underground ruins was, IIRC, explained in the tier 4 cutscene. There is no need to provide them if it means costing dilithium as in the asteroid mine. The exchange has been full of cheap junker suits ever since Nukara was added.