I don’t have a Fritzbox to test but it seems the documentation from AVM describes a roadwarrior scenario. You need a site-to-site VPN to have full access to any device from both sides.
I didn’t find any documentation about setting up site-to-site in the Fritzbox user interface. In this case I think you have to use a cfg file and import it to your Fritzbox (Add VPN button).
Check the link posted by @dnutan to get an example of a cfg file for the Fritzbox and a working /etc/ipsec.conf for Nethserver.
Here is another cfg example (in German).
There’s a “Configure FRITZ!Box VPN Connection” tool to create cfg files.
Then you need to set same values on both sides as @pike described here.