This is mentioned in the v4 breaking changes document:
https://www.chirpstack.io/docs/v4-breaking-changes.html#gateway-to-chirpstack-mqtt
I know that the WisGateOS2 does support ChirpStack v4, but I don’t know if your gateway model (already) supports it. If it is not supported, then you could:
- Try to install the ChirpStack MQTT Forwarder on your gateway (many RAK gateways are based on the same MIPS architecture)
- Use the ChirpStack Gateway Bridge to accept UDP data