Zerve

Hello there, I'm in the post-game right now (tier 4) and the underwater New World door to Latias/Latios isn't appearing... I tried to look for an answer, but haven't found anything as of yet. The "instructors"/quest givers aren't there anymore and all the other quests of that tier are completed (exluding Zygarde). Any idea how I could resolve this issue? Thank you. Kind regards, Zerve