But i have a problem: when you send the message, you do not send a simple String, it is more a char buffer with the size of a String or something right?
Wrong. Forget that the stupid String class exists. What you send, and receive, is an array of bytes.
A byte is the same size as a char, so you could think of sending and receiving arrays of chars. But an array of chars is NOT a String, and does not have a substring method.
You could make the array of chars into a string (which is NOT the same thing as a String), by adding a NULL terminator in the proper position in the array. That's why the number of elements in the array is required on the sending end and provided on the receiving end.
Do NOT use Strings in your code. Learn to use strings, instead.