cant maintain a high temperature in glass cage

[CatandDiallo]

The reptile store guy told me to place the thermostat probe above the substrate, this way the thermostat will make the uth heat up the cage entirely until its 92 where the probe is.

It's still not the right thing to do. That's how you end up with a snake with a burnt belly (if the snake digs under the substrate and lays on the glass, which they frequently do).

My thermostat is zilla from petco.. Like ~ $50

That'll be an unreliable thermostat. That's why you're having problems. I would return it.