I’m not sure I understand your setup.
The way OpenVPN works (or most other IPSEC/VPN technologies) is you set up a server. A lot of routers/firewalls will have an OpenVPN server built in. You need to expose the OpenVPN port to the Internet (I don’t remember the port off the top of my head) and have a dyndns or static IP. If you are hosting the server on a computer behind your firewall you need to set up port forwarding.
There are tutorials online for how to create the various certs and client config files but usually if you have a gateway or firewall that supports it there will be a wizard that asks a bunch of questions and spits out those files for you.
Presumably, you would run this VPN on your home Fritz.Box or on a machine behind that box in your main home.
On your remote machine in the cottage you install an OpenVPN client on the machine(s) you want to be accessible to/from your home network. Take the config file the wizard generated or you created by hand and pass that to the OpenVPN client and have it connect. The client will reach out over the network to your home OpenVPN server and establish an encrypted connection.
At this point, the remote machine can see all the machines on your LAN and all the machines on your LAN can see your
remote machine. However, one thing to be aware of is the OpenVPN LAN is separate from your LAN and from the remote LAN. This means that your remote machine will have a different IP address in a different subnet from your local LAN and that IP is unlikely to be the same as its IP on it’s LAN. I believe there is a way pin certain clients to certain VPN addresses but, for example, your home LAN is 192.168.1.x and your cottage LAN is 192.168.1.x your VPN LAN might be 10.0.8.x.