The instructions use the basic_pkt_fwd == basic packet forwarder. This forwarder allows sending to one destination. You need to compile poly_pkt_fwd and use it to allow for multiple destinations. (At that moment the config you are referring to does work, for TTN it's better to use the one with TTN frequency plan)
If required at the RPi branch of the lora_gateway code is the version of the library preconfigured for RPi with MultiTech MCard I used to compiler poly_pkt_fwd. (The version you compiled earlier should work as well.)
You should enter a unique value for the gateway_ID, not an empty one.
