Configure 450 for subscriber gateway

Hi folks…
being new to the MikroTik world i have a load of questions, ill try to make this one as brief as possible.
We are a WISP and want to run the mikrotik routers at “lit buildings” to send internet access to end user suites.
What we wanted to do was limit bandwidth to each Ethernet port to that which the subscriber is paying for.
What is happening is that we have two 450’s and one allows us to limit both tX and RX on each port, while the other one does not. It only allows us to limit one or the other, not both. Why cant we do this ?
Second part of the question: is there a better way to achieve this and if so, how?
thanks all!!