Unfortunately, if you signed up for an account directly through your Fire TV and did not provide MotorTrend (MT) with your email address (the email address associated with your Fire TV account is not shared with us due to privacy reasons) you will not be able to watch MT on any other device. MT customer support will be unable to assist you as we will not have any internal records of your account. Your account information is stored within the device you subscribed on. There is no way possible to connect our system to your device after the fact.
In order to remedy this situation the only option is to cancel your current subscription, and then re-sign up entering your email address or through a desktop/laptop computer.