Who said the humidity should be at least 70%?
You don't want it to drop below 50% for sure, and 50% would be the minimum, 60% is more than fine. There are several things you can do to raise the humidity, but you've left out key information.
Tank size, bedding, water dish/size, what kind of bulb are you using the lamp?
Different substrate can hold moisture better making it easier to maintain higher humidity, Putting a larger water dish can help raise humidity slightly, but that will depend on how big of a tank you're using as to what size bowl you can fit. Heat lamps suck up humidity, and depending on if you're using an actual bulb or a CHE, that may not be the ideal setup.
Also why is your UTH on the side of the tank?