Gauge thermometers are notoriously inaccurate to begin with. And then, they will only measure the temperature exactly where they are placed...so if it's stuck on the side of the glass, that is what it is measuring. You'd have to actually lay it down directly over the UTH on the floor of the cage to get even an inaccurate reading of that from the gauge.

You can get a ~$10 dimmer switch for table lamps from any hardware store. It's like an extension cord with a dimmer switch connected to it. You have to be careful and monitor your temps carefully, but it can work...especially in a room where the local ambient temps don't fluctuate too much. Definitely save up for a reliable t-stat, though. Makes things much safer and you'll have more confidence that your equipment is behaving as expected.