You might want to enable logging of dropped/rejected packets in your OpenWrt firewall and use wireshark/tcpdump to watch the traffic.
However, please keep in mind that the security provided by PPTP is fairly low. The tutorial on corz.org also mentions this fact.
What are your requirements regarding VPN throughput, client support and user account management?
What is your router hardware?
Do you have a spare public address (IPv4 or IPv6) you could assign to the VPN server?
Would you consider setting up the VPN on the OpenWrt router itself?
Would it be feasible for you to use a different VPN technology such as IPsec, OpenVPN or wireguard?