Lag while streaming iTunes movies on iPad Pro

I have fibre optic broadband and have no issues with Netflix or Amazon prime while streaming. However, when streaming my iTunes purchased movies on the iPad Pro there is stuttering, freezing and lag. I checked them on the Apple TV and they are fine on that. I have reset all settings on the iPad, turned it off and on again, 'forgotten' the iTunes account then signed back in, did the same with the online details, changed the DNS to 8.8.8.8 and reset the modem and router, but to no avail. Eventually I did a full restore through iTunes while it was plugged into the Mac, but still nothing. When I spoke to Apple they weren't very helpful and said that it seems like an "issue with your ipad server connecting to the apple server for itunes purchases", but didn't know how to fix it. They wanted it to go back to Apple, but I think it's a setting issue somewhere and a new iPad would have the same issue. Can anyone help?