Wait, just the official Wattpad app handles offline reading okay-ish now? If you're relying on it, there's a huge caveat – you have to manually mark each chapter for download BEFORE you go offline. It doesn't auto-cache your current book's whole thing, which has screwed me over on the subway more than once.
I keep hearing about unofficial third-party apps from sites like APK mirrors that promise full-book downloads, but they're a major security risk and violate terms of service, obviously. Honestly, the better move is to find authors who publish their work on multiple platforms, then use a dedicated e-reader app like Moon+ Reader to handle the EPUB files offline. Way more reliable.