얼마나 is "how _" as in to what extent, like how much. 어떻게 is a methodical "how"

얼마나 걸려? - how long does it take? // 어떻게 해? = how do you do it?

In this sentence if you said 치킨은 어떻게 싱거워? it sounds like "how is it possible that the chicken is bland??" as if you were the cook.