After testing multiple sources (APKMirror, APKPure, and official backups), the most stable and verified version is:

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: Go to Settings > Security and toggle on "Unknown Sources" to allow installation of apps from outside the Play Store.

Android 5.1.1 was released in 2015. While it was a stable operating system, modern app developers have largely abandoned support for it to focus on newer APIs that support advanced video rendering and security features.