P2P should work, no tricks needed.
EE have been known for blocking ports but this would normally be outbound to stop you hosting services like port 80. If I understand correctly your EE side is the client?
Also if I follow correctly you seem to have no issues from the client using the WAN IP? This ruling out blocked ports.
Set the secondary DNS to itself? Just set NVR DNS1/2 to a trusted external DNS provider.
I recall a thread a while back of something similar with new Hik units?
I remember having a inkling it could be a Double NAT issue due to ISP (CGN) especially knowing the companies mentioned (EE, Sky VM etc)
You would need to traceroute your DNS to diagnose that. I think this is what the guy from EE is trying to explain in simple terms.