Rune Seeker Questline stuck Akt3

Hello, I have already reached the endgame, only to realize that I haven't completed the Seeker questline. In Act 2, I was missing the Skull of the Titan; in Acts 3 and 4, I had completed everything else, but without being able to finish Mystic Refuge or Plunder Island (as the necessary NPC was missing). Even after finishing Act 2, Acts 3 and 4 still cannot be completed. Is it possible to reset the questline or complete it in some other way? As a result, I am missing out on important seasonal mechanics.

Greetings