Maybe you have to use readRaw() and parse the string yourself?
From https://github.com/mathworks/thingspeak-arduino/blob/master/src/ThingSpeak.h
| * @brief Read a raw response from a public ThingSpeak channel |
| * @param channelNumber Channel number |
| * @param URLSuffix Raw URL to write to ThingSpeak as a String. See the documentation at https://thingspeak.com/docs/channels#get_feed |
| * @return Response if successful, or empty string. Use getLastReadStatus() to get more specific information. |
| * @remark This is low level functionality that will not be required by most users. |
| * @code |
| void loop() { |
| String response = ThingSpeak.readRaw(myChannelNumber, String("feeds/days=1")); |
| Serial.print("Response: "); |
| Serial.print(response); |
| delay(30000); |
| } |
| * @endcode |