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.
We updated our Apple TV app recently, and this will provide you an opportunity to recover that account. When you open up the app it will present you with an option to subscribe or sign in. Pick the subscribe option, create a MT account with your email address, and then when prompted to pick either monthly or annual, there will be a restore subscription option below that. If that does not work, MT customer support may be able to help if you provide the email address for your MT account you just created and your Apple ID.
At this time on the other platforms, 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. Unfortunately, since MT does not have any record of your initial transaction there will be no possibility of refunds or credits due to this issue.