If you have already opened port for your VPN, I think the problem is not related to the NAT, but you can try with the PPTP ALG enabled and disabled. And may I know the exact topology of your network?

I guess your purpose is to block certain https website. We can try to use Access Rule as a workaround. First, you need to know resolve the IP address from the URL of the websites and than set rules...

I think this Forum only supports English communication. you can contact the local support for help.

For your first question, how do you configure the Online Detection? Automatically or Manually? If you set it Automatically, the router will ping the WAN IP and look up the DNS of the WAN. If this...

You can go to Firewall-Access Control-- Access Rule to block some certain ports, but since the p2p connections usually use random ports so it is hard to block and may affect some other applications....

Wifi? But the 5120 has no wifi function. What is the topology of your network? And is there any error message in the log of the 5120?