@afreedman, The UPN you shared in the format "email#ext#@subdomain.onmicrosoft.com" is for the guest users. To help you better, we would need to understand, what type of user account you hold in the tenant.
is a global admin role being assigned to you?
When you try to login with default account, what is the exact error that you get?
Do share the information so that we can help further.