Really Simple SSL pro has the option to force the use of 2FA (two factor authentication) to protect your accounts from unauthorized access when your password is stolen. If for some reason you are unable to receive the required 2FA codes you will be locked out of your website.
For example: You don’t know or have access to the email address of an admin user or have lost your phone with the TOTP app.
To disable 2FA in a contant can be defined in your wp-config.php.
Add the following line to your wp-config.php file
define( 'RSSSL_DISABLE_2FA', true );
(This disables the Really Simple SSL 2FA checks on the login page)
After logging in and resetting your 2FA settings, you can remove the line from your wp-config.php again to re-active 2FA again.