Try to mist more lightly or for a shorter duration, as 85-90 is too high. Horned frogs (except from the Amazonian horned frog) are not tropical but more sub-tropical. Some authors have even suggested that Chacoan or Ornates can handle humidity between 35-60% but to be safe I keep mine near 60% for the most part.

So have you got another heat lamp yet? You're right, as you've had it for 2 months I take it's a baby and at this stage it's essential for it to be fed every 1-2 days but I can't imagine a week or so of less than ideal conditions should do any harm.