The answer to my question is simple. I just removed all Port-Forwards and instead of using DynDns-hostnames to access the local devices I am now using the local-IP’s. So ones connected with VPN I have full access to the LAN and thus there is no need for any Port-Forwarding.
On my ASUS RT-AC51U I did the following:
Configured some port-forwarding-rules to local devices that I want to reach from the Internet.
Setup OpenVPN-server
Both work fine and I have access to the local devices with and without the usage of VPN.
Now I’m searching for a possibility to gain only...