char buffer to String in Arduino UNO
Do not use String. And especially do not use String in the way you are using it.

You can read more about why not here.

Instead stick to just using char* buffers.

The correct way of doing what you want is to define the buffer outside your function and then pass that buffer to the function to be populated:

int getAPIData(char *buf, int maxlen) {
  return inet.httpGET("myserver.com", 80, "/call.php?id=1&amp;q=1&amp;code=1001", buf, maxlen);
}


Then you call it with:

char *my_buffer[450];
getAPIData(my_buffer, 450);
Serial.println(my_buffer);