There’s a desperate lack of documentation on how to create achievements for Android in Unreal Engine.
The tiny bit I was able to find personally mostly just directed people to the Flappy Bird project.
Even if asking people to reverse engineer someone else’s code to figure out how something is done were reasonable, the achievement implementation in that project is still incredibly messy and has a lot of extraneous steps.
In the end, reverse engineering that code was exactly what I had to do. This was an absolutely insane process, so to save others from doing this I’ve created a guide myself on how to add achievements.