From the FAQ: What if I lose my phone? Many websites require a backup two-factor authentication methods such as SMS and TOTP (authenticator 6 digit-code apps)

From the FAQ: What if I lose my phone? Many websites require a backup two-factor authentication methods such as SMS and TOTP (authenticator 6 digit-code apps) even if you are using a U2F security key such as Krypton. For certain sites that allow U2F only (such as Google Advanced Protection), we recommend having a backup phone with separate Krypton U2F key setup or a physical hardware key that you store securely in somewhere. We are actively building a robust account recovery service with partners to solve this problem and make U2F/WebAuthn a viable "single-factor" login system. We hope this will remove the need for these backup methods that make your account vulnerable to phishing attacks. We also see this as a major barrier to wide adoption of U2F/WebAuth/2FA so we are eager to solve this problem.