That wet substrate will become a bog and breeding place for bacteria (should be damp so it clumps on fist; but not drip water out). If fogger can't be adjusted; recommend not use it in the critter keeper. Also, if you use distilled water in fogger (recommended) that could create another problem for frog with distilled water soaked substrate.

You can actually prevent humidity loss by partially sealing the vents with aluminum foil taped to top. Try sealing back and sides first. If need to, you can seal up the front vents until 80-90 percent of all vents are sealed. That shout help maintain your humidity between misting .