Have you contacted Medicaid?

The money for Medicare is for Part B which are office visits and outpatient. If you are receiving Medicaid coverage still with SSI benefits, then hypothetically you can either cancel Part B and keep Part A Hospital for Medicare then use Medicaid for office visits or you can see if Medicaid will cover the part B premium costs for Medicare.

Medicaid is weird because it is often a state run program so things differ.

You may also want to call Medicare and see if they have any programs to help pay the premium. As I recall, there are some options out there depending on income.