If the homestead is under attack, then it is by design that no portals are opening. You need to stop the fireballs raining down (doing something in the northern part of Luculla). Talk to Zixzax and check your quest log.
If Zixzax is by a portal encouraging you to enter, then did you do so, get a conversation with Astarte or the demon (depending on the room) and then return to the main area by the portal (rather than using Rift Travel to exit the End of Time)? In some cases, you need to return to the main section of the homestead with a main character in the lead; for the Hall of Heroes, for example, that is required to get a dialogue triggered, which is needed to completely open that portal.
Are you sure you checked the last room opened? If Zixzax is by the statue it might not be clear which portal was opened last, if you didn't check it right away, for example.
If Zixzax is standing near a portal or the entrance to the homestead, approaching the next unopened portal may trigger a conversation and get the portal opened. If he isn't near a specific portal, just get close to each of the unopened ones (aside from the Forbidden Zone portal, which doesn't get opened at all).
If Zixzax just isn't registering that a new portal should be opened, then either teleporting him or killing him may reset the problem, and start him running around the portals opening whatever ones you have the inert stones to open. That wouldn't help if you didn't meet anyone in the last opened room, though, or you did but they were unresponsive. If applicable, the best way to kill Zixzax, if you are a high enough level (with D:OS that was about level 12 or 13, depending on party build) is to first make sure he doesn't have any help nearby; teleport him a couple times if he is close to the weaver. Spread your characters out on all 4 sides of him, so he can not fireball everyone at once, and then have them take turns attacking him, so you get his health down as much as possible before triggering combat. Summons help absorb some damage (he prefers attacking party members, if he has a clear line of sight), but the weaker ones don't do much damage if he isn't stunned or knocked down (which is he is fairly resistant to, though is susceptible to tripping).
If you have not gotten far enough to trigger the attack on the homestead, doing so and then stopping the attack can get Zixzax opening portals, even if he had a problem before then.