Killing Zixzax could help with an issue in the Classic version of the game, but not in the Enhanced Edition.
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 that is not the case, where is Zixzax? Beside a portal, or the statue, or sitting near the weaver of time?
He may be standing beside a portal waiting for your characters, or beside the statue, in which case you may need to talk to the statue or enter the last opened elemental portal.
Does the last opened portal have a marker on the map? If not, it wasn't completely opened. You need to enter, proceed far enough to get a cutscene conversation with Astarte or the demon, then return to the main section of the homestead through the portal. If there are any more portals to be opened at that point, Zixzax will move to the next one.
If that doesn't help and you are playing on PC, please email supportdos2@larian.com with your save (or email/reply/PM a Dropbox or Google drive link, etc).
Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin Enhanced Edition\PlayerProfiles\<ProfileName>\Savegames_patch' folder.
In Windows Explorer you can zip a file or folder by right clicking on it/them and selecting 'Send To | Compressed (zipped) Folder' (and similarly in OSX/Finder).