What are you sending and what are you getting back, lets start there.
You have this, n = readString.toInt(); but you need to convert it AFTER you get everything
so right I think it is only getting 1 char then it goes to the IF/ELSE and then gets cleared. You don't give it time to get multiple chars.
you need a stop char for it to look for like ":" or ";" or "."
If the incoming char is anyone of these, then it know that you have everthing and it should then convert the string to an INT.