If you are not 65 then are you on Medicare for special condition or are you on Medicaid? Will insurance companies be required to accept you during the initial enrollment period if you are already on Medicare now as you would not be the typical person enrolling at age 65. Advantage plans have a network of doctors, to go outside that network is to pay out of pocket. And not all clinics take Advantage Plans...for example Florida and Arizona Mayo don't. In Minnesota some insurance companies have made a deal with Mayo to have Mayo in their network. But every year networks change.
https://www.investopedia.com/articles/personal-finance/010816/pitfalls-medicare-advantage-plans.asp.