Agnostic Atheism is the Agnosticism you're talking about. They mean the exact same thing, both work.
a·the·ism
ˈāTHēˌizəm/
noun
disbelief or lack of belief in the existence of God or gods.
By...
That is how it works though? Agnostic can be a noun or an adjective. You can call yourself an agnostic, or you can use it as an adjective to something else. Gnostic means knowing, A means Anti. Agnostic means Anti-Knowing, you don't know. That is literally how the word is created...
ag·nos·tic
aɡˈnästik/
noun
1.
a person who believes that nothing is known or can be known of the existence or nature of God or of anything beyond material phenomena; a person who claims neither faith nor disbelief in God...
Agnostic is an Adjective. If just means you don't know for sure.
You can be an Agnostic-Christian, saying "I don't know if God really exists, but I believe he does."
It just means you lack the evidence to prove in full if your stance / belief is correct.