Thanks for request. I have my answer.
I am sure that frequency plan is US915. Every join request is seeing by the gateway. I am using LoRa Open Server by broocar and packet forwarder by Semtech. In my solution downlink frequency is always the same as uplink (from logs). For EU868 is good by for US915 is not. Whether the server sends the data to the gateway, determines the frequency of the downlink transmission? If yes LoRa Open Server is not for US915. There are no frequency settings.