Can't connect to VPN Server

[ThreadStarter]

Hi all,

I am trying unsuccesfully to connect to the VPN Server app, but no luck so far. I hope anyone can help!

The Basics:
NAS: ASUSTOR AS6302T
ADM-Version: 3.4.4.RAT2
VPN Server-Version: 1.0.2.r241
Router/Modem: AVM Firtz!Box Fon WLAN 7390
FRITZ!OS: 06.85
Client: MacBook Pro 2019
macOS Mojave 10.14.6
VPN Client: System
Internet: DSL via fixed IP-address
*other* VPN-Setup: there is already a VPN connection configured using the Fritz!Box as VPN Server. However this doesn't work for all outside conections because of the difficulties of mixing IPv4 and IPv6 architecture.

The Problem:
I can't conect to the VPN Server...

What I did so far
- installed VPN Server App on NAS
- started PPTP and L2TP/IPsec connections
- added privileges for an existing user and an existing group for both protocolls
- left the configuration in L2TP/IPsec as is
- configured the client as follows:
-- server address: the fixed ip address from our provider
-- account name: the account i created for this and added the above mentioned privileges
-- entered the shared secret from the configuration
- activated port forwarding in the router to port 8000 tcp protocoll
- checked open port with online testing

ERROR: I can't connect to the server

I am not sure about the VPN Server configuration. The local IPs are 192.168.0.100 to 192.168.0.199 given via DNS from the Fritz!Box. I changed the configuration in the VPN Server app the dynamic IP addresses from 192.168.0.200 to 192.168.0.254 - and the local DNS-Server from 8.8.8.8 to the adress of the Fritz!Box. Is this correct?

I still get the same error and I am not sure where I'm going wrong. Any advice/help would be appreciated!

Thanks in advance!
Christian