LoRaWAN PHYPayload does have FRMPayload

hi
i use Laird gateway and setting mqtt forwarder

Why should it have a FRMPayload? It is a join-request. Please refer the LoRaWAN specification for more details: https://lora-alliance.org/lorawan-for-developers

Thank you for your response. how to configuration lora app server to Join-accept message my node