This is my network structure:
The Computers on the internal network are setup as 10.0.0.x/24 as well as the definition of the network on UTM9.
Behind the UTM9 firewall there is the Fritzbox, which is the gateway to the internet. For the FritBox I have assigned a 10.0.10.x/24 network.
When I define a new firewall rule under Network Protection, to connect the 10.0.0.x/24 network to the 10.0.10.x/24 network in both direction for any IP protocol, should it than possible to connect to the FritzBox from the internal Computers? Also the IP-Phones need the connection to the FitzBox!
Before I have installed Sophos, I had a working VPN connection to another Fritzbox. If I setup also an additional firewall rule under Network Protection for the 192.168.178.0/24 network, should than again work this VPN?
Of cource, the above rules must be placed before the internet connection, so that Sophos UTM will the handle first!
I ask these questions, because I am not able to test it with the risk, that the connection to the internet will be disconnected, because I am runing an private Web-Radio and I don't want to interrupt my listeners!
Thanks a lot for your support!
** Summary **
Due to the fact, that it is a lot to read, I try summary it.But first, many thanks to Phillipp for his help and patience.
It is important, that the internal network 10.0.0.0/24 and the Fritzbox network 10.0.10.0/24 are indepandant and also all clients in this networks have a 255.255.255.0 mask!
Therefore in the Fritzbox the DHCP-server has to be enabled, that clients on WLAN will get a IP. For that I have for the DHCP-Server set the gateway to the Fritzbox, but the DNS to an DNS-Server inside the internal Network.
In the Fritzbox under Home network->network properties->Static routing table you have to setup a entry for the internal network and the gateway has to be the external Sophos port on the Fritzbox network.
Finally in Sophos you need under network Protection->Firewall->Rules there is an entry necessary, which allows the traffic between the internal network and the FritzBox network.
Than everything should work, including WLAN.
For the VPN I have setup a new VPN connection between the external FritzBox and Sophos.
of course these need to be two separate networks, because this is on two different interfaces of your firewall! Otherwise it would be difficult for the linux kernel to decide the routing between…