I'd suggest looking into apps like Crunchyroll Manga. They sometimes have Berserk. Also, keep an eye on Kindle. Sometimes manga publishers make their works available there too. Just search and hope for the best!
I'd recommend Tachiyomi. It's a popular choice among manga readers. However, make sure to download it from a trusted source as there might be unofficial or modified versions that could pose security risks.
I'd suggest looking into the Crunchyroll Manga app. It sometimes features Attack on Titan. Also, keep an eye on official manga distribution apps like Shonen Jump, as they might have it. But remember, availability can vary based on licensing and your location.