How many portals are open, besides the observatory (where you first entered the homestead)?

On the right, past the statue and before you get to the tapestry of time (where the weaver sits), there should be an open portal. If you have not done so already, enter, speak to an NPC seated at the table (about half way down, on the opposite side [with the default camera position]), then return to the main section of the homestead through the portal, with a main character still in the lead. If that was the issue, you should be able to follow Zixzax around, opening a few more portals.

If you did that, look for an open portal that does not have a map marker. Flags are placed when a portal has been completely opened, so one without a marker means that is the one holding up Zixzax from opening the next.

If Zixzax is beside the statue, try speaking to it, to see if you can choose an elemental portal to open (the first is after the Hall of Heroes and Inner Chamber, the 2 portal sites just past the statue).