How use already working FritzBox VPN with UTM without modifying the Fritzbox

Question

It would be a destination problem to change anything on the Fritzbox on the other side. 
 The VPN connection was working without any problem, as long the FritzBox was in the local network. All devices on the network on the other side of the VPN connection was simple to connect via the IP address and vice versa. 
 Is there any way to foreward all adresses of the to he FritzBox without setting up a Sophos VPN connection?

PhilippRusch · Answer

Hello, 
 is the Fritzbox working with a transfernet in front of the Sophos UTM? This transfernet should be different from both local networks you used before. 
 The this is only a routing problem: on the Fritzbox you have to define a route to the local LAN behind the Sophos UTM. This is to be found at "Network Settings". 
 On the Sophos you have to define a gateway route to the remote LAN at the other end of the VPN-Tunnel with your local Fritzbox-IP as the gateway. (static route) 
 Finally, you need to setup firewall rules on the Sophos-UTM to allow the traffic you want.