There's a lot of apps out there: this post has a pretty good list.

I'm personally using Memmy, which feels somewhat similar to Apollo to me. It's also currently the only one I'm aware of that has reached the official App Store (as opposed to being in Testflight like the others). I found it slightly confusing at first to add new users to it, and you won't be able to create new accounts on it (although this might just be universal among all apps), and when you click on Lemmy links it'll go to the browser instead. Otherwise, it's worked very smoothly for me so far.