Let’s start by reversing the string

jnu37tl817st

Rot didn’t give anything, let’s trying Atbash on both letters and numbers

qmf73go293hg

Go is not a valid keyword. We need to find the opposite of go since we used Atbash. Stay is the keyword

qmf73stay293hg

That’s an excellent range for a Polybius square.

Using the table below convert each pair of digits to it’s corresponding coordinate

fomeigeigdifficultsixsixthrjt

fom88difficult663jt

Bit of an easy one… Split the code in 2

ec64neo

kw596nr

Reverse both strings

oen46ce

rn695wk

Regroup

oen46cern695wk