Have just bought myself an asus DSL-AC68U to replace a bt hub 5 so I can use nordvpn. I`ve managed after some issues to get it all working apart from the likes of bt sport. It works with the vpn deactivated, but once active it no longer connects via the youview + box.
Why would you expect it to work with the VPN active, as it needs a direct connection to the BT YouView multicast servers?
Its not distributed as a unicast stream like BT iPlayer etc. Its a receive only multicast stream which is sent to all receivers, there is no return path, its a bit like a TV transmitter.
Just turn off the VPN when you want to use BT YouView,