Complete the quest Journey to Cavalon, acquired from Jamei in Maatu Keep, to gain access via Zos Shivros Channel. There is an alternate roundabout way to enter Cavalon. From Zos Shivros Channel, exit into the Boreas Seabed explorable area and head west into Mount Qinkai. Then, turn south to go through Breaker Hollow (or start from here if already unlocked), to the Archipelagos. From there, head east along the northern edge to enter Cavalon. Note that you still have to complete the above quest if you want to continue the campaign storyline.