Using VPN with a draytek behind a zyxel firewall


i am trying to install a draytek device only for VPN behind a zyxell zywall usg200.
If i connect to the draytek from intenal lan i can access vpn and authenticate.
But if i connect from outside, i cant connect. So i think the Zyxell blocks it?

I inserted the following:

    incoming: any
    Source: NAT Internal IP
    Destination: NAT internal IP
    Service: GRE and pptp (1723)

    Interface: WAN2
    Original IP: NAT Internal IP
    Mapped IP: Draytek IP
    Port: PPTP (1723)
    Protokoll: TCP

Similar settings work fine with RDP or https. But i canĀ“t access VPN. I deactivated everything of VPN i found in zywall.
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John HurstConnect With a Mentor Business Consultant (Owner)Commented:
Did you try NAT Traversal on the Draytek?  You have created a double NAT type of situation.

Can you use the Draytek as firewall also and eliminate the other firewall (in other words, why so complicated?)
loosainAuthor Commented:
Our customer wants it so. I have a new question started with a little more detailed information:
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