If you subscribed to AMC+ via the Roku Channel, then you can watch AMC+ from the Roku Channel. You can't watch it via the AMC app.
While subscribing to content via Roku is convenient, this restriction/limitation is often frustrating to users who would like to be able to watch via other means. That's why I now only subscribe directly to services, and will not subscribe through Roku again until they change that behavior.
DBDukes http://www.dbdukes.com/Roku Ultra (4660) Roku Streambar (9102) Roku Streaming Stick+ (3810)
That's why I now only subscribe directly to services, and will not subscribe through Roku again until they change that behavior.
Roku is not likely to change this as it is the same with many (all?) 3rd party services/platforms. I suspect it may be a requirement by all channels when your subscription is with the 3rd party instead of the channel itself because you don't actually have an account with the channel.
Like you, I only subscribe directly with premium services on their websites. This not only allows me to view the channel on all the platforms it supports (computer, mobile devices, various streamers), not just Roku, but minimizes the finger pointing (channel >> Roku and Roku >> channel) if/when problems occur.
Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T