Yeah I'm not sure - it's very confusing however, I've had a breakthrough - I just started a 30 day free trial of amazon prime and have got US content video on iPad and am able to airplay to samsung tv so all is good. Also will get other prime benefits of kindle books and music etc if I keep it going.
The samsung amazon app doesn't like my vpn stream from the mac though so have to airplay from iPad.
Thanks for your comments/help though dazhann, this dialogue may help others in future trying to get it to work.
Obviously theres a difference between amazon prime and primevideo.