How do I use my subscription through apple to log in to the amc+ app outside of appletv?

You can’t watch on the AMC apps or website if you subscribed via Apple TV. You have to watch within the Apple TV App. If you subscribe directly with AMC then you could watch on www.amcplus.com or the new AMC+ apps. 

See: https://support.amcplus.com/en/support/solutions/articles/11000107087-how-do-i-subscribe-through-apple-tv-