I think that the Ape Titan is a hibernating Titan from underground of Connie's village.
He could not have been someone from within the Walls or else he would have known about the current tech and such. Likewise, he couldn't have been from outside of walls, because he would have known about the Scouting Legion and how they can definitely kill Titans using their 3D gear. Also if he were from outside as a Titan, he would have been spotted a mile away, but instead he just magically appeared out of nowhere within the walls. And if he were from outside, he would have to get past the walls somehow. We know he can climb, but even climbing would leave noticeable marks, and we know not a single trace of breaching was found.
So if he's not from within the walls or outside the walls, he had to be from underground. One idea in mind was that he's involved in some epic mission to dig a massive underground tunnels to reach inside the Walls to destroy humanity. This, however, would make no sense since he could have just invaded upfront and wipe out humanity np.
The only explanation that makes sense to me is that he is one of the Titans that have been hibernating for a century(s) like those titans within the walls somewhere and have just recently awakened. This fits the description that he doesn't know anything about current events and standards. We have proof that Annie is capable of crystalizing herself, and we have proof that there are Titans within the walls themselves that are hibernating and are capable of reawakening too, as feared by Pastor Nick. Armin speculates this:
And so far in the series, Armin's speculations or hunches have been 100% or pretty close to 100% correct/relevant.
Then Reiner and Bert's original mission could very well have been to search for the dormant titans or wait for their reawakening.
Also, One of the first places he was heading was Castle Utgard, which according to wikia means "Stronghold of the Giants," and the place is already pretty mysterious since there are really deep dungeons that I don't think anyone's been.
If the Ape Titan was hibernating and has just reawakened, then it looks like he may not be the only one. It looks like a lot more dormant Titans are about to reawake as well, and hell's gonna break loose.
One thing that doesn't quite make sense with this is that how could he be the "mastermind" behind all this? But one thing is for certain, the Ape Titan cannot be the one who commands Reiner, Bert, and Annie. I believe that Reiner, Bert, and Annie's faction are looking for the Ape Titan or his kind and that was their original mission.