Unable to compare char from radio (RF24)
That is not a good way to solve it.  That is a hack.  The String class does bad things on microcontrollers and it is something you should try to stay away from.  What you need to compare two char arrays is the strcmp function.  

if(strcmp(someCharArray, anotherCharArray) == 0){
    // the strings are the same
}


Use google to find out more about strcmp.