I found a solution how to solve this.
You just need to set the SendTimeout on the binding of the client endpoint of the router. When creating the proxy the router will set OperationTimeout=SendTimeout on it's channel.
// add the endpoint the router uses to receive messages
serviceHost.AddServiceEndpoint(
typeof(IRequestReplyRouter),
new BasicHttpBinding(),
"http://localhost:8000/routingservice/router");
// create the client endpoint the router routes messages to
var client = new ServiceEndpoint(
ContractDescription.GetContract(typeof(IRequestReplyRouter)),
new NetTcpBinding(),
new EndpointAddress("net.tcp://localhost:8008/MyBackendService.svc"));
// Set SendTimeout, this will be used from the router generated proxy as OperationTimeout
client.Binding.SendTimeout = new TimeSpan(0, 10, 0);