Health "insurance" woes.

I had to find a new ACA insurance provider for 2016, because my 2015 provider dropped. The lowest-cost provider covered the asthma doctor that I must see once per yer, but not my PCP, so I figured I could go see the asthma doctor and find a new PCP if I needed one. I saw my asthma doctor in May, and also took a breathing test in his office (both scheduled at the 2015 appointment). Fast forward to April 2017, when I am sent a bill for $1,700 for the breathing test. I'm thinking "WTF, this doctor is covered, shouldn't anything done in his office be covered?" I call this insurance company, and they tell me I needed "prior authorization." I submit paperwork for both an internal investigation and the authorized ACA external investigation. I heard nothing back from either investigation, and the bill is now in collections. What are my options?

(This provider dropped out for 2017, but by then I quit my job and had a low enough income to have qualified for the state insurance.)