The two subscriptions are different. If you had the subscription to the one on The Roku Channel it wouldn't be good for the stand-alone channel/app. Usually the stand-alone channels/apps have more than what's available through The Roku Channel, so I'm surprised your show isn't available, but I don't think there's anything you can do about it short of dumping the one subscription and then subscribing to Discovery+ in The Roku Channel. Then you'd probably find something that you can only watch on the other channel.
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