sprintf

char url1[] = "/demo_sim908.php";

sprintf(frame, "visor=false&latitude=%s&longitude=%s&altitude=%s&time=%s&satellites=%s&speedOTG=%s&course=%s", lat, lon, alt, time, satellites, vel, course);
Serial.println(frame); // Output the content of frame
String url7 =  String(frame);
String nurl1 =  String(url1 + url7);
Serial.println(nurl1); // outputs nothing
const char* nnurl1 = nurl1.c_str();
Serial.println(nnurl1); // outputs nothing