To root your Galaxy S5 mimi on 5.1.1 you need to flash a custom kernel first to the device which will set SELinux to permissive, so that you later install SuperSU to manage root permissions.
However the folks at XDA didn’t took much interest in developing a 5.1.1 custom kernel for the galaxy S5 mini. But thanks to Blue cat, a developer who took the matter into his hands and released a custom kernel S5 mini 5.1.1 release so that the users could easily root it.
You can grab Blue cat’s custom kernel from the download link below, and simply flash it on your S5 mini (G8ooH) using Odin.
Once you have installed the custom kernel, flash SuperSU zip via TWRP recovery to get root access.