I actually bought two of those 600w thermostats about a month ago and like them a lot for the price. Did you set both day and night temperatures on the thermostat? I'm betting you only set one and that's why you're getting low temps.
Ambient temp around the cage should be 80ish. The hot spot that you get from the uth should be 88-90f. You need to measure the surface of the glass to get that temp, not the substrate.
I actually set both. For the first time in my life I actually read instructions, only bc I couldn’t figure out how to toggle between the two. Lol.