TDU's servers has always had problems with NAT. Even though it may be disabled, I think it's to do with the actual router itself. I had a pretty good router once, fast as you'd like, but it would not work with TDU. I simply switched back to my old BT Voyager 220v and that worked like a charm. Instead of throwing the router away, I simply connected the other router via the ethernet ports and disabled the DHCP server on the secondary router (Seeing as you're 2nd line IT tech, I shouldn't have to remind you not to plug the RJ11 into the wall ;))
So, the solution that worked for me was to use a different router. But seeing as your router is a very expensive one, perhaps this may not be the desired solution for you. How about trying a different router, just to see if it is the router the problem is coming from, then putting the expensive router back in...? At least you'll know if it is the router or not that is causing the problem.
If you don't have a spare router lying around, do a quick test by putting just your gaming/TDU computer in the DMZ, to see if a required port or other feature on the router is blocking TDU's outgoing/incoming connections.
Let us know if this worked or not :)