The http integration of The Things Network is limited in the sence that it does not allow to only forward the payload_fieds ( https://www.thethingsnetwork.org/docs/applications/http/#uplink), so you can pass it directly to Thingspeak. In order to fix this, I used a Wemos D1 to do this filtering/proxy between The Things Network and Thingspeak. I am sure this will work on other devices too, as long as they run the Arduino IDE and have some network connection.

This code can be compiled using http://platformio.org/

The ArduinoJson library ought to install automatically, but if not, install the ArduinoJSon libary:

pio lib install 64

Then put your wifi credentials in lib/credentials/credentials.h, compile and upload the code to a Wemos D1. You can test the functionality by navigating with your browser to http://$wemosip/

The code forwards only the payload_fields part of incomming json posts to /ttn to Thingspeak.

A simple test is to send some data to the Wemos. Make sure you have created a channel on Thingspeak first and have the Write API Key around.

read -p "Wemos D1 ip: " wemos_ip
read -p "Thingspeak API key: "  api_key
read -p "Value to post: " value

DATA=$(cat <<__EOT__
{ "foo" : "bar ",
  "payload_fields" : {
    "api_key" : "${api_key}",
    "field1" : "${value}"
  } }
__EOT__
)

curl \
  --header "Content-Type: application/json" \
  --request POST \
  --data "${DATA}" \
  http://${wemos_ip}/ttn

If this works, configure your home router to forward packages a port of your choice to port 80 of the Wemos. Last, configure the things network http integration to point to http://$yourip:$yourport/ttn

The firmware also allows for over the air (OTA) updates. Adjust the upload_port in your platformio.ini file to match the ip of the Wemos.

If you end up with "400" responses, the buffer for the json parsing might not be large enough. Check your JSON on http://arduinojson.org/assistant/ to get the size right for your settings.