The Server marks down the time your name and IP when you first enter. It then checks how long it has been since that point, and if /register does not work, it probably means that too little time has passed.
If you’ve waited multiple days already, that must mean that your IP is constantly changing.
If your join date is very old, /register will also not work, but I doubt this is the case for you.
However, if this is the case, simply create a new character of a different name and try to /register with it instead.