How to setup a private network

I plan to change my gateways communication network from Wifi to LTE. So I have to control the amount of data my gateways forward, because I will pay for it.
I have started to set my devices and gateways to private network, but I can’t find any other settings for personalizing my private network. My gateway actually forward every private network LoRaWAN frames, not only mine.
Is there a way to identify my private network ?
Thank you,

If you have a NetID, then this value can be set in your chirpstack-network-server.toml config and it can be set in chirpstack-gateway-bridge.toml for filtering on NetID, so that only uplinks matching your NetID will be forwarded to the ChirpStack Network Server.

Thank you for your answer,
Currently I have not a NetID.
I am not sure it answers my question. The aim is to make the gateway send less frames to chirpstack.
If I am not wrong, chirpstack-gateway-bridge is not part of my gateway but chirpstack

Device -> Gateway -> GW Bridge -> Network Server -> Application server

I want to act on 2nd arrow. I think your suggestion act on 3rd, right ?

A LoRa gateway has no knowledge about which are your devices and which are others. The only way is by filtering uplinks on DevAddr based on the NetID you are using, which is what the ChirpStack Gateway Bridge can do for you. Note that by default 000000 is being used by ChirpStack.