If your iPad is running iOS 5.1.1, it is an original (first generation) iPad, for which Apple formally ended update support in September 2012. iPad1 is obsolete.
The final IOS release for iPad1 was iOS 5.1.1. It cannot be updated to any later major version of iOS as it lacks the minimum hardware requirements needed to run more recent versions.
Many App Developers have also dropped support for older devices and iOS versions preceding iOS11/12 - and have withdrawn older versions of their Apps from the Apple App Store. With removal of compatible versions of Apps from the App Store, your original iPad1 will have very little usefulness or remaining utility.
If Apps that you need to use now have higher minimum iOS-version requirements, your only option is to replace your iPad with newer model. Even the basic current iPad model supports current iPadOS 13.x - and has hardware specifications that will vastly outperform your original iPad.
Sorry, this is clearly not the news that you were hoping to hear.
I hope this information is helpful - if only in clearly stating your current options and potential ways forward.