I agree that the experience of adding subscriptions is plagued with problems. When I first got my Stream box in July, I was able to add UHD and Premier Sports without any difficulty. However, we are looking after a friend's 1 year old for one day a week soon and I wanted to add Kids TV but it keeps saying there is a technical glitch - try again later. I would suggest that you do what most help desks say in times like these - switch off the box, then switch it back on again 30 seconds later. Sorry I can't be more helpful.
