String Class Weird Behavior - c++

Question

I'm new to Arduino and C++, and I'm working on an Arduino project for school. I have a Firebase Cloud Function and I want to call if from the Arduino. I start a connection with the server and give it a query:
const String query = "QUERY";
if (client.connect(server, 80)) {
Serial.print("Connected to ");
Serial.println(client.remoteIP());
// Make an HTTP request:
client.println(query);
client.println("Host: SERVER-HOST-NAME");
client.println("Connection: close");
client.println();
}
Now this works very fine if the query string was declared as one piece:
String query = String("GET /cloudFunctionName?dataThatShouldBeVariable=FixedData HTTP/1.1");
Example:
String query = String("GET /updateDatabase?temp=10 HTTP/1.1");
When using the method above the cloud function triggers and the database gets updated. But I can't use it in production because I have some variable data so what I did was:
String query = String("GET /cloudFunctionName?dataThatShouldBeVariable=" + String(VariableData) + " HTTP/1.1");
Example:
String query = String("GET /updateDatabase?temp=" + String(10) + " HTTP/1.1");
Now when I print the results of the two strings I get the same result, but when using the second method the cloud function doesn't get triggered and the database doesn't get updated.
I believe that the problem is with the query variable not with the server or the hardware but I couldn't find an answer. I've been working on this for hours and I really got mad and I don't know what to do. Please tell me what is the problem and how to fix it.