Many thanks for your responses.
I tried to setup a private routing environment and change the gateway sending his data to this backend. But I couldn’t find any chance to change the server address except of the router. But I think its not the right way. I used
ttnctl gateways edit <GATEWAY_ID> --router-id 192.168.2.121
But I dont receive any up- or downlink at the bridge. After rebooting the gateway it has problems with the config.
Is there any way to use the official TTN gateway in a private environment? It has to be a private one because the TTN broker (I think) drops the the packets. 
(I asked the question allready in TTN gateway central)