The cannot run iOS 5.1.1. Its official software support ended with iOS 4.2.1 , which was released in November 2010.

Downgrade and dualboot status of almost all iOS devices · GitHub

Since official updates ended over a decade ago, the device is largely a "legacy" item. You can still use it for:

For devices that can run it, iOS 5.1.1 (Build 9B206) was a stability-focused update released in May 2012. Key features included:

Many users mistake the 2nd generation for the 3rd generation, as they look nearly identical. However, the 3rd generation (32GB and 64GB models only) can run up to iOS 5.1.1. What iOS 5.1.1 Included (For Supported Devices)