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ISP :

I've set up the following configuration.
ISP - Drei Hui Tube LTE (Internet Modem in Bridge Mode) acessed to DECO M5 in Router Mode. On the LAN Port I have attached an Synology NAS on which I've running an L2TP VPN Server.
The following Ports are forwarded to the NAS 1701,500,4500. If I'm connected to the DECO WLAN im able to start a VPN Connection although the VPN Server is accessed over the dynamic DNS Adress. If I'm outside of the DECO WLAN the Client got 'Server not responding error'
Has anyone been able to setup such connection and can give me a hint what is wrong or missing here. Or is it possible to set up a logging on the DECO device. I'm familiar with networking and have set up a lot of differnent devices in the past, but in this case I'm clueless.
Is there a firewall running on the DECO or is it possible to configure IPSEC and VPN Passthrough such like on other devices.

I will be grateful for any hints.
thank you
best regards
Andi