I set up a Microsoft 365 Developer Program account and can't sign in

Question

I set up a Microsoft Developer Account and used the instant sandbox option. I can now sign in to my regular Microsoft account and see the subscription is active and it shows the admin username and the domain as being 27l6kp.onmicrosoft.com. When I try to go to the subscription I am given "This username may be incorrect. Make sure you typed it correctly. Otherwise, contact your admin." I used my business Microsoft account which has it's own domain, but we use Microsoft for the email. I am not sure if this plays a role in there being some wires crossed somewhere or not, but it's my best guess. I tried using a variation of our domain and the onmicrosoft.com to see if I could find the right way to login, but that didn't work either.