RAK811 cannot recieve any user downlink

I am planning frequency at AS923 profile. The node (iam using rak811) joined to chirpstack server with rak831 gateway easily but the downlink created with restapi still not recived. I checked log at gateway it showed some thing below, and the tx led was on when send downlink!

I don’t understand why?? because when joinning network (otaa/abp) the downlink is received to confirm the node join successfully.

The different things i seen is datarate: the downlink with SF10 is not recieved, i cannot change to SF7 or SF8 because it totally base on app server. Anyone find the solution for me :frowning:

is your endnode Class C ? if not, uncheck it in device profile.

Thanks, i changed device to Class C and fixed this problem.
However, in Class C, the maximum downlink lenght i sent is 4 byte, the longer data lenght will cause “Unknown network error” at node! Moreover, my node doesn’t send any ack to server and server only try to resent onetime !

Hello here.
I’m a new leaner of chirpstack.
Please can anyone tell me how to set the server so as to downlink an endnode ?