heya! There are two tablets in that area, both use the same simple substitution cipher (same cipher key). http://practicalcryptography.com/ciphers/classical-era/simple-substitution/
Basically write down the coded messages and start figuring out substitutions for the letters. It’s easier to guess letters for smaller words and then intuit larger words and letters from there.
When you know some of the tricks to cracking a simple substitution cipher, it really is not terribly difficult! :) Here are a couple of links that tell you how to generally solve simple substitution ciphers: https://www.dummies.com/games/cryptograms/cryptography-101-basic-solving-techniques-for-substitution-ciphers/ and https://www.instructables.com/id/How-to-Solve-Simple-Substitution-Ciphers/ -- If you wrote down the messages from both tablets, it should actually make it even easier to ultimately create the cipher key needed to decode them. As to why it's "hard" is because one of the two areas that are locked behind the gates there contain one of the single most powerful items in the game. Also, these are optional - the way out of level 3 and to level 4 has nothing to do with the cryptographs :)
