I have a Samsung 4K tv and have the same issue. I can stream live content OK, it is just the replays of older content that won't stream. To get around the issue, I have plugged an amazon fire 4k stick into the TV, and it streams all the content fine. So I think it is something with the app on the samsung TV, and unlikely to be a WiFi or router issue.
The tv checks for updates automatically and seems to be running the most up to date software.