Help with my under tank heater?

I have a small zoo med repti-therm uth. I plugged it in too a Zilla thermostat to regulate the temperature. I got it from Petco for about $50, and it seems kinda cheap. If you go on their website and type in "Zilla temperature controllers" you'd see it. So my problem is, no matter what I have the temperature on it won't drop below about 94 degrees. I had it set to 90 but my temp gun said it was almost at 100! So I brought it down a few degrees but it still stayed in the high 90's. So I took it all the way down to 80, and my temp gun says it's still at 94! I can't figure out what to do!

(side note: the temp gun isn't broken)

I`ve been using zilla thermostat for almost 4 years without any problem so far. When it works stupid, just unplug and plug again. It should be working good or it`s defective.
Don`t forget. You`ll have to set the thermostat a little bit lower than temperature you want.