If you just forgot your password, I think it's a simple matter. You just need to reset the password on the login screen, and they'll send you a verification email to reset your account password.
However, if you forget your email or login credentials, you can contact the service directly. However, I don't think that makes much sense, because you'll likely always remember the email you used to register. Furthermore, the email is tied to your phone, unless you lose all access to it, including your phone, due to loss or other reasons.
I don't think an email is always tied to a phone number. It depends on emai service providers and their policies on account registration and updates later. Even Gmail years ago did not require any phone number for creating an gmail account. There are still email service providers with privacy and you can create email accounts without any personal information.
As a user of Duelbits or any platform, it's too difficult to lose all information you have with your account for its registration like email address, password, phone number (if any), 2FA and other things but I believe if you lose all of such things, you can do advanced KYC and get your account back. Surely this process of account recovery takes a longer time as the company can not make mistakes to give access of that account to hackers.