I think this has to do with your device connectivity or something on that end.
To confirm, can you try logging in to ThingSpeak, creating a new channel and ingesting a single point into the channel using your browser? Instructions on ingesting data into the channel from a browser are in the "API keys" tab of your channel (or, see documentation here https://www.mathworks.com/help/thingspeak/writedata.html)
If it works from the browser, you can focus your investigations on the device end.
