Hi Andrew
You need to change the Gateway here from 192.168.188.1 (The IP of your Fritzbox) to the IP of your NethServer…
Otherwise, any contact from the VPN to another box in your LAN will get replied to. Only your Fritzbox will not send the reply back over the VPN via your NethServer, but send it towards the Internet, and your provider will be discarding private packets…
After all, your boxes have only the gateway of the Fritzbox, not the NethServer as Gateway…
My 2 cents
Andy