phone bill

I've have tired many time to change the date of my bill to the 28 th of each mth  but will not let me  as bill date is the 21 st of each mth I alway pay on the 28th and every nth go through can I change the date to 28th and it never does so now once again I have a restriction on my phone until the 28tb when I've tried to change bill date on my app it will never allow me to !! Frustrating is not tbe word