Change 16 to 17 in the /opt/ttn-gateway/packet_forwarder/lora_pkt_fwd/global_conf.json file and see if the issue is resolved.
…
It is possible that ttn modified some parameters and caused us915 to have this problem in join. We have fixed this issue and will release it in the next version of the gateway firmware.
Unfortunately, this is only a temporary solution, as the 16 that TTN (sometimes) requests might very well be 17 (soon) again. Please see ERROR: Packet REJECTED, unsupported RF power for TX - 24 - #13 by arjanvanb and please add a link to that topic, and to Downlinks rejected by gateways for certain TX powers · Issue #2106 · TheThingsNetwork/lorawan-stack · GitHub, in the RAK forum as well, so RAK knows what to expect.