Your first step -
Properly identify the model of device you actually have (you used mixed names in your post).
Compare the first four characters of your serial number with the entries in this table:
https://wiki.mobileread.com/wiki/Kindle_Serial_Numbers
Then re-read the title of the post you quoted - before you edited the title.
See?
Whatever you have, it isn't a K3.
I do expect you will have to make more changes than just editing the post title to make it work.