God is defined as a "maximally great being" whose existence is necessary. A being who necessarily exists is a being on whom everything else is contingent. God would be such a being. God is the creator of everything who alone eternally exists. Therefore he is a necessary being.
Everything's existence being contingent on God is true only in the mind of a theist who believes God exists in the first place. Your belief that God exists means that you're prone to the presupposition that everything is contingent on him. That everything is contingent on him is not fact, nor is his existence proven.
God is an abstract idea, and the modal S5 argument Plantinga uses could be applied to any abstract idea:
"Brahma, Vishnu and Shiva are necessary because the existence of everything (in a Hindu paradigm) is contingent on their existence. Thus, Brahma Vishnu and Shiva are necessary beings".
That's logically fallacious because there's no proof Brahma, Vishnu and Shiva are even real. The modal S5 argument is often used for mathematics, but we know maths, we use maths, we understand maths and maths ''exists", proven so. God has none of those qualities. We can't verify his existence in order to be able to assume him as an inherent thing able to be used in tangible logical ways.
It's all apologetic, abstract, logic-bending nonsense.
If you claim that it's possible that a necessary being like God exists then you are claiming that God exists in some possible world. But, being a necessary being, if he exists in some possible world he exists in all possible worlds. Hence, he exists in the actual world.
Brahma, Vishnu and Shiva might possibly exist, because we have no way of knowing that they don't exist, and as an open minded person, I'm open to the idea that anything is possible if we take the view that universes can exist that exist in different states to the one in which we exist. That doesn't mean Brahma, Vishnu and Shiva exist, it means I'm open to the idea that they exist. Being open to it is far from it being fact. I'm open to evidence. Read: The Anthropic Principle.
Arguments from the possibility of universes that exist different to the universe in which we exist are by their very nature, not applicable in this universe.
God is not a "necessary being" any more than Brahma, Vishnu and Shiva are.
Any person whom you define as necessary would essentially be God. You're more than welcome to claim that God is a leprechaun. Plantinga's Ontological Argument makes no such claim.
Plantinga's Ontonolgical argument is an argument starting from the assumption that the idea of "God" has logical merit in an argument. Yet "God" has no discernible, tangible, inherent "realness" like, for instance, an equation or a number does.