My family member had the exact same problem. It went on for months, uploading huge amounts of data when all devices was turned off and while they were all at work. I took my own initiative to troubleshoot this myself as Vodafone told them that their computer was infected. Well.. after a testing devices at a time (by having them disconnected a few days at a time) the problem still persisted.

I then reset the router back to factory default, reconfigured it and enabled firewall. Since that very minute, no more huge upload data was being transferred.

Then.. to our surprise, all four ethernet ports died and I could not get any computer connected to the router. D-Link deemed it fault thus we got a replacement.

For us, two things... router was uploading possibly due to the DNS attack as mentioned above.. or.. the router was really on its way out and did something that was surprising. I would go with the former.