Do markeplace plans have to conform to certain standards like available covered providers?

I'm asking because I have been having some major issues with my United Healthcare plan from the marketplace. I learned earlier this month that the doctor I switched to specifically to have an in network doc at the beginning of this year is now out of network. Now I need an MRI but I learned there is only ONE covered provider within a 100 mile radius and that's half an hour away. To be clear, I don't live out in the middle or nowhere. I live in the second most populated city in my state and the richest county in the state with hospitals and doc offices on every other corner. I'm also needing to see a pulmonologist, neurologist, and a hepatic surgeon. I'm potentially about to embark on a journey of life altering chronic illness and I am terrified that I will either not be able to find providers or stuck with only one option even if the doc sucks or isn't the right sub specialty. What do marketplace plans have to adhere to and what are the consequences if they don't? In America do we have any rights as healthcare consumers to certain kids of care or discolsure about major limitations in plans based on location? If you could point me to any reading or good place to inform myself I'd really appreciate it! May UH rest in distress.

submitted by /u/rainbowsforall