For simplified login situations (e.g. not requiring two factors for authentication) you'll probably have some kind of password or password-less (email) login fl

For simplified login situations (e.g. not requiring two factors for authentication) you'll probably have some kind of password or password-less (email) login flow where you can login at first and then enroll the FIDO2 device. Otherwise there is no way to login from any other device. An alternative is that you can authenticate a new device from an existing device along with a re-auth on that device to verify it's them. Discord has a feature similar to this with it's QR login system.