Since you aren't using the Play Store, you must allow installation from files.
The experience of using Messenger on Android 5.1 is a study in functional compromise. Modern iterations of the app include high-definition video calling, augmented reality filters, and complex encryption protocols that demand significant RAM and processing power. On a 5.1 device, these features can cause the system to lag or crash. Consequently, many users in this ecosystem opt for "Messenger Lite." This streamlined version of the APK is specifically designed for older versions of Android and limited hardware. It prioritizes the core functions—text, photos, and basic voice calls—while stripping away the "bloat" of stories, games, and heavy animations. This ensures that the device remains responsive without sacrificing the ability to stay in touch with friends and family. Messenger Apk Android 5.1