Try proving this by assuming to the contrary that is composite, or 1.
In my(poor) opinion to prove this we can't do what mr. melese offered...
Here is a link I think it be useful! Conjecture 17. The Ludovicus conjecture about the Euler trinomials
I let where and and .
Since , I let .
I ended up with , which led me to nowhere, and I decided to post the question.
I found your technique quite interesting. I see why you chose . It makes sense since . That's quite creative.