Ok I had issues enabling dev mode for series x. I followed the instructions above but developer mode didn't show up. So two things:
One is that I was holding the buttons down as I pushed them. Just pushed each button one at a time and let it go.
The second thing is that I think I'm confused as to which are my bottom buttons and which are my top buttons. I pushed LT RT LB RB. Maybe I'm backwards but I was thinking that the buttons on the top would be the ones closest to the other buttons and the bottom ones would be the ones closer to the battery compartment.
Anyway, try both ways.