cannot connect to internet with dialup modem (hotspot-ppoe)

I have a dialup modem connected to the serial port of a RB411A.
I would like to know how to catch the ip given by my ISP for connect with my clients(via hotspot or PPPOE). My problem is I can’t translate the ip of the ISP from those I give to my clients. I make Nat masquerade to serial interface. I can make a ping to internet througt routerboard but I can’t from computer.
Any help will be apreciated.
Thanks in advance.
Xevi.