You have a "Kindle Touch", software updates for that Kindle model stopped at 5.3.7.3. So you can't update to a 5.11 since that doesn't exist for you Kindle model. Similarly how Kindle Paperwhite 3 (aka Paperwhite 7th gen in Amazon's nomenclature) supports ended with 5.16.2 or PW1's with 5.6.1.1.
This is the jailbreak thread and method that applies to your device and its current firmware:
https://www.mobileread.com/forums/sh...d.php?t=186645
Before re-doing jailbreak you should probably first try to just install the latest hotfix bin (from the snapshots thread) using "Update Your Kindle" option and then check if KUAL and KOReader run correctly.
So, you'd device is already at the latest (a last it ever got) firmware for that model.
And 5.11.x is newer firmware than 5.3.x.y ( 11 > 3) so installing that (if it were ever possible for your Kindle model, and it isn't) would not be a "downgrade" but an "upgrade".
Since there is a working jailbreak method for your firmware there isn't even a need to do any sort of downgrade.