This may be the solution to your problem:
/ip firewall nat add action=dst-nat chain=dstnat dst-port=1723 in-interface=Ext-IF protocol=tcp to-addresses=192.168.9.5
/ip firewall filter add action=accept chain=forward in-interface=Ext-IF protocol=gre
/ip firewall filter add action=accept chain=forward out-interface=Ext-IF src-address=192.168.9.5
Have a look at this topic: http://forum.mikrotik.com/t/pptp-forward-port-not-working/58722/1