You said you were already 66 years old; Medicare starts at 65 - so you are a full years late in signing up.
Where did did you get the bad info ?
Medicare.gov - Medicare & the Marketplace
Did you not get any info from your ACA insurer that the ACA plan and any premium tax credit subsidies you might be getting to help with your premiums would END when you turned 65?
Because of what happened, there is no way that Part B will begin before July 01 - you will just have to pay out of pocket if you need to see a doc before then or any other Part B covered services.
Actually, that may be just one important matter. Another is the Medicare
Part B premium penalty that you will forever have to pay. And you may have a hard time getting a Medicare supplemental , if you want one, because underwriting will be in play.
You cannot even join a Medicare Advantage plan or I think, a Medicare Prescription Drug program until you have Part B.
from the above Medicare.gov link:
Once you’re eligible for Medicare, you’ll have an Initial Enrollment Period to sign up. For most people, the Initial Enrollment Period starts 3 months before their 65th birthday and ends 3 months after their 65th birthday month.
Once your Medicare Part A coverage starts, you won’t be eligible for a premium tax credit or other savings for a Marketplace plan.
If you kept your Marketplace plan, you’d have to pay full price. For this reason, in most cases, you’ll want to end your Marketplace coverage once you’re eligible for Medicare. You may get a notice from the Health Insurance Marketplace that says you may soon be eligible for Medicare & can change your Marketplace plan. Don’t wait for the notice. Be sure to sign up for Medicare once you’re eligible.
Hope you don't get a big surprise when you do your taxes this year (for 2019). If you were getting an ACA premium tax credit.
I am sorry this has happened to you. How did it happen cause there should have been some failsafes. When did you lose the ACA coverage?
Healthcare.gov - Changing from Marketplace Health Insurance to Medicare
It's Always Something . . . . Roseanna Roseannadanna