Private Medicare Advantage plans sold by insurance companies achieve their "benefits" in part by limiting their network to contracted providers.
Some phyicians/facilities will not accept lower reimbursement and are not contracted with the private plans.
You may want to consider switching to original Medicare, but your costs/premiums may be higher. And you may or may not be able to add a Medicare Supplement.
Medicare Advantage plans can be a financial benefit and provide services not paid by OriginalMedicare, but you must receive treatment from its contracted providers.
Best wishes to you.