Because iOS 5.1.1 is ancient, you cannot sideload using modern tools like AltStore or SideStore (they require iOS 9+). You have three options:
If you downloaded Subway Surfers on this Apple ID back in 2012: subway surfers ipa ios 5.1.1
Unlike the modern version which features complex 3D models and aggressive ad implementation, the iOS 5.1.1 compatible IPA features: Because iOS 5
1. "Installation Failed" (AppSync Issues) If the IPA fails to install, it is almost always because you do not have the correct version of AppSync Unified. The modern version of AppSync on the default Cydia repos often drops support for iOS 5/6. You must find a legacy-compatible version (often listed as "AppSync for iOS 5.0+"). The modern version of AppSync on the default
2. Crashing on Launch If the game crashes immediately after the "Kiloo" logo, the IPA is likely too new. Subway Surfers dropped support for the ARMv6 architecture (iPhone 2G/3G) early on, and eventually dropped ARMv7 optimization. You need an older version of the IPA, roughly version 1.0 through 1.8.
3. The "Time Travel" Exploit On this version of the game, the "Daily Challenge" system relies on the device's system clock. If you set your device date forward by one day, you can instantly unlock the next daily challenge. This was a famous exploit in the early days.