This happened to me on a new level 50. I found another thread that stated that you needed to complete the quests in Boralus up to and including the point where you choose your first questing zone at the scouting map, I think it’s called “A Nation Divided.” As soon as I completed that (despite having done the Nazjatar chain and the first cloak quest already), the portal back to Boralus appeared in Silithus. So, even if you don’t plan to quest there, complete this quest!