RAK7289C Chirpstack 3.x (JSON) downlink parse error

Hi all, I am trying to use RAK 7289C in packet forwarder mode with protocol ChirpStack 3.x (JSON) format sending to a remote MQTT server.

I have tried to follow the payload format provided by the ChirpStack docs for v3 commands posted on their website, however it appears that there is an error parsing my payload.

Firmware: WisGateOS v1.3.9.

I am getting the following error when sending this generic downlink payload:

  Mon Jan 29 23:30:02 2024 daemon.err gwBridge[4094]: gwBridgeParseMosqDownlinkJSONV3: Failed to get txInfo obj
  "token": 34520,
  "gatewayID": "rB8D//4aiXA=",
  "items": [
      "phyPayload": "BxACsGJJbyGoGJ+9CRnn1GzZLc+kJRtHAx3qWbIL6Lc6R0nuvFpY6yb88bbsplyKJxBOg8/hYr0w9Pzq",
      "txInfo": {
        "frequency": 923000000,
        "power": 14,
        "modulation": "LORA",
        "loRaModulationInfo": {
          "bandwidth": 125,
          "spreadingFactor": 9,
          "codeRate": "4/5"
        "board": 0,
        "antenna": 0,
        "timing": "IMMEDIATELY"