Creating a PPTP VPN Server on your DDWRT enabled/hacked router might not work for you. This is usually because GRE is blocked although the TCP port for PPTP is enabled.

You need to run some custom commands on the router at each boot to allow the GRE traffic.

Under Administration -> Commands

/sbin/insmod nf_conntrack_proto_gre
/sbin/insmod nf_conntrack_pptp
/sbin/insmod nf_nat_proto_gre
/sbin/insmod nf_nat_pptp

Add that to a startup/boot script. Save.

Found the information at

They have two extra lines, but they were not needed on my setup.