The uth does NOT heat the air so it is NOT helping your ambient temps. With the thermometer probe laying on the substrate you are NOT measuring the ambient temps, you are measuring the surface temp over the uth. If that is getting over 100F it means the floor of the tank is probably at a dangerous temp. If the snake burrows, which they can and will do, it can be burned.

What size tank are you using?
What is the room temp where the tank is?

I have a 40 gallon tank is a room that stays 68F. I have to use a 100w che and a 60w bulb 24/7 to keep my ambient temps correct. You may need to use both lights depending on the tank size and the temp of the room. Until you get a tstat the uth should not be used.

You need to figure out what the actual temps are on each side of the tank before we really know what needs to be changed. Unplug the uth and move the probe to a position where we can find the real temps of the tank. It is best to measure the temp an inch or two from the substrate to see what the temp is in the zone your snake stays in.