Nowadays there are two methods to set up Fritz!Box VPNs:
a.) Through the box' UI (Less options but easier)
b.) Through an external (Windows) program called "Fritz!Box Fernzugang einrichten" (German) or "Configure FRITZ!Box VPN Connection" (English)
Both methods have in common that they use "IKEv1 Aggressive Mode", which StrongSWAN considers weak.
Once you enable "IKEv1 Aggressive Mode" in StrongSWAN, it changes its name to "WeakSWAN"
The good news is, that when you create the config using method b you get plain text config files to be imported into the Fritz!Box and these config files can be modified before import, e.g. to use "IKEv1 Main Mode".
In order to connect my Linksys WRT1900ACS using StrongSWAN on LEDE to two remote Fritz!Boxes, I first went the weakSWAN way to verify everything works, then I modified the VPN configuration for the Fritz!Boxes to use main mode and disabled aggressive mode in StrongSWAN again.
And yes, the Fritz!Box uses a PSK (Pre-Shared Key), so do most WLANs too.
The main benefit of the IPSec way of doing the VPN in the Fritz!Box is, that this method is widely supported in other devices too.
You can find instructions on how to connect to the Fritz!Box' VPN for
- Android
- Windows Phone
- Blackberry
- Small-dick-Fon (iPhone)
- Windows
- Linux
- macOS
- Cisco ASA/RV320 <-> AVM FRITZ!Box
- pfSense
- Lancom 1681V/1781V <-> FritzBox
- and many more
In turn that means that
any of the devices which can connect to the Fritz!Box VPN should also offer the same connectivity to the other devices in the list.
So if Lancom routers can connect to Fritz!Box VPN and the Fritz!Box VPN can be connected from Android smartphones or iPhone's excuse for a smartphone, the iPhone excuse for a smartphone should also be able to connect to a Lancom VPN.
This means it would be helpful to know which kind of router you currently own on both sides of your desired connection, maybe they already have a common VPN support.
Sadly no E2 image known to me has the neccessary modules to be a VPN client to a IPSec/Fritz!Box VPN on its own.