Going back to my posts over a year ago. I was informed by BBC that only those Roku model numbers ending in EU or UK were recognised by Iplayer. I think things may have changed since. But, back then, BBC had to share their content with the rest of Europe (hence models ending EU). Now UK is no longer part of EEC, so I'm not sure what the situation is. But at the time I was told it was to protect paid content being available for free. Strangely, I've since bought a Roku stick model 3810X and that works fine with Iplayer.
