I think the TL-ER604W can not do this yet.

May I know what kind of PPTP VPN do you use and the topology of your network?

I think the P2P connection is very difficult to block since it use random ports to set up the connection. And the APP block function may not act very well in doing this as well.

May I know what kind of PPTP VPN that you set up? PPTP LAN-to-LAN or Client-to-LAN VPN? And what is the topology of your network?

Why you are so sure that it is the ER6120's fault? From your description, I think there may be some factors on the Cisco side that block the traffic from the ER6120. Firstly, just think about again,...

There is a FAQ about your problem, hope it can help you; If you still have the problem, you can contact the supporter for help directly. http://www.tplink.com/en/faq-1003.html