You can use the following command to see which channels are available for your node
Than you can use the following to enable a certain channel, for example of you want to add channel 4:
Than check with at+get_config=ch_list, it should say ok.
Try this and see what happens
And if you have ADR enabled it will transmit on whichever is best I guess.
Let me know how it goes
Ah here is the manual by the way:
http://docs.rakwireless.com/en/LoRa/RAK811/Software_Development/RAK811 LoRa AT Command V1.5.pdf