quick answer: (imho) For OH2 KNX1, Routing is generally better and it does not occupy a tunnel interface (gateways support up to 4 usually).
Try Tunneling for now to see if the issue goes away. Then try to troubleshoot Routing (if you fell like it).
There is some info here also: HowTo: GIRA KNX/IP Router
Much more on google of course 