I have the same problem and I did the same steps and I have a Hisense Roku TV.
But it works on my ipad and my computer and on another non-smart TV with a NowTV smart stick in the same house.
So how can it be a BBC problem?
It's a BBC problem because they are the ones that created and support the channel. You can't compare the iPlayer on different devices because they are all programmed in a different language, usually by different developers. Sounds like they have better developers for iOS devices. I don't know what a NowTV stick is, but it's certainly not using the same software as a Roku.
Roku does not write the channels for other providers, and they have no way of fixing a problem with a channel. They don't have access to the code. If everything else is running fine on your Hisense TV, then the problem is with the specific channel, in this case iPlayer. And it can only be resolved by the company that provides it, BBC.
If BBC is saying it's a Roku problem, they're simply pushing off their problem on someone else. Keep on them, as only they can fix it.
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It’s every time you click on the app to try and watch them, same error message every time. Tried deleting and reinstalling along with clearing the cache, still same problem every time on all of the apps