So I set up my VPN server in my SOHO so I can access my openmediavault machine and files from on the go. I set up port forwarding on the router to forward a port to my VPN server ... as you would. After I set the ports I was able to try the setup from a remote connection and it all works fine. However, after a couple of hours (couldn't narrow it down more) the connection keeps failing. So I jump into my router configuration again and after I edit the port setup, it works again. 

So it seems, that the port forwarding seems to stop working after a while ... did any of you experience similar behaviour and maybe offer some ideas? This makes my router kind of useless.