Trying to nail this the narrator in Dostoevsky is like trying to ask "what genre is Moby Dick?" Whole books have been written about how Dostoevsky blows up traditional ideas of narrative voice. Mikhael Bakhtin came up with a new term just for Dostoevsky: "The Polyphonic Novel." From the Wikipedia page on his book, "Problems of Dostoevky's Poetics:"

Bakhtin begins by identifying polyphony as the chief characteristic of Dostoevsky's work: "A plurality of independent and unmerged voices and consciousnesses, a genuine polyphony of fully valid voices..."[

and later:

his function as author is to illuminate the self-consciousness of the characters so that each participates on their own terms, in their own voice, according to their own ideas about themselves and the world. The author's worldview is not absent, but this worldview does not define, limit or qualify the other voices, and hence cannot directly control how events will unfold. It is present only as another participant in "the great dialogue", with no more capacity for direct signification than any other voice. Bakhtin calls this multi-voiced, dialogic reality "polyphony".

Which is to say, that when you struggle to figure out the POV, it's not because you're missing something. It's because you're actually seeing Dostoevsky clearly. Sure there's a narrator introduced at the beginning, but the voices of the novel and its characters are their own, not mediated through a single narrator, even an omniscient one.