Those analog things are nothing but junk. I was going to suggest if your humidity is really that low, even after checking with a digital gauge, that you might consider just water substrate, period, and use a TetraFauna Reptofilter/waterfall (works in as little as 2" of water). You could give the frogs rocks and landmass areas and places to climb on as per usual, and if you are worried about crickets drowning you could try feeding in a separate container or with tongs. Definitely don't cover up more than half the tank up top, these guys need good ventilation, and watch out for melting plastic wrap if it gets near the lamps!

Did you say you have an 18"x18"x24" Exo-Terra...? How many WTFs of what age/size do you have in that? That's a bit shy size wise... (I probably read this in another of your posts elsewhere but I should have gone to bed an hour and a half ago and I'm super tired, lol).