If you have the white NOW TV box or the purple Roku streaming stick then you will have to buy something more up to date.
I have both of those and they are both unusable now. This is because they have a slow processor in them that can't cope with modern apps.
I recently bought a Roku Express, which was released in Autumn 2017 I think, and it works great as it has a much faster processor. It is quite cheap to buy on Amazon.