I had a similar problem with 2 brand new Humax HDR-1100S 1TB Freesat boxes. Internet was absolutely fine on ethernet and wifi. But no On Demand apps would work except Netflix (which showed that the internet was fine).
Spent considerable time on the phone to Tech Support who recommended restarts, different connections then suggested a broad set of (implausible) explanations (e.g. router blocking sites despite many other devices accessing them fine!). Ended by recommending returning the boxes!!
After extensive trial and error, eliminated problems with the boxes (by trying at a friends house). Internet searches suggested changing the default DNS Server on my router. Changed it to Cloudfare 1.1.1.1 and 1.0.0.1 and bingo all worked fine again!
