If you are looking for information because you are an ATT retiree, please read this article first: https://medicarequick.com/att-retirees-information-about-medicare/
However, I want to answer your question about whether you can purchase a drug plan from AON and purchase a Medigap plan from someone else, and still get your benefit.,
As of 10/30/2020 The answer is still YES. You can purchase the drug plan from AON and purchase your Medigap Plan from someone else.
If, however, you purchase your Medigap plan from AON, and purchase your drug plan from someone else, you will NOT get your benefit.
Be sure and read the article that ATT Retiree Louise Fulton wrote just for you.