You need to take over the battle tower and get your dragon form before you can reach the teleporter that will bring you to the place you can shut down the barrier.
When you defeated Amdusias in Maxos temple, a corridor there (across from the chamber with the bedrooms upstairs) that was blocked off by a force field became available. It leads to a teleporter than will bring you to Sentinel Island, where there are a few quests to do and areas to explore before you take over the battle tower.
BTW, your first encounter on Sentinel Island you can mindread to go up a level, so the higher level you are the more of an experience bonus that will be. You don't need to get to a high level, but it would be better if you got to a new level before entering, to get a full level bonus. If you are pretty close to level 22, there is a beach in the fjords you can swim over to with a few Black Ring members on it, if you have not cleared it out yet, otherwise double check if there is anything left to do in Broken Valley.
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