This sounds like the same problem as this one.. So, just regurgitating the reply from AWS support below:
The actual process as you describe, requires you to log-into the account with root credentials, complete the sign-up steps and then remove the account from the Organization. Since, the email no longer works, I would suggest you to reach out to the Accounts & Billing team via a support case to check if there is a way to change the root email address on the account.
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